Introduction to the difference and usage of single quotes and double quotes in php_PHP tutorial

The article uses a large number of implementations to introduce the usage and difference between single quotes and double quotes. Students who need to know more can refer to this article carefully.

In PHP, usually a string is defined within a pair of quotes

Follow the steps below:

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$out = str_replace(array('rn', 'r', 'n'), '', $out);PHP 提供三种定义字符串的方法：单引号、双引号、本地文档（英文叫做 here document 或者 heredoc）。

$out = str_replace(array('rn', 'r', 'n'), '', $out);PHP provides three ways to define strings: single quotes, double quotes, local documents (English called a here document or heredoc).

Single quotes: Using single quotes is the most efficient method because PHP does not check built-in variables and escape sequences in single-quoted strings. The only characters that need to be escaped are backslashes and single quotes themselves.

Double quotes:

Built-in variables and escape sequences are checked, but escaped single quotes are not recognized. This also shows what is wrong with the code at the beginning. The correct approach is to use double quotes to define the escape sequence for newlines:

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$out = str_replace(array("rn", "r", "n"), '', $out);

Local documentation:

Check all built-in variables and escape sequences, double quotes do not need to be escaped. For example:

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echo this is a "here document" example. just for test.

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echo this is a "here document" example. just for test.

EOT; Simply record it to deepen your impression.

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'I am a string in single quotes' "I am a string in double quotes"

, such as:

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'I am a string in single quotes' "I am a string in double quotes"

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"I am not a valid string since I have unmatching quote marks' 'Me neither!"

The PHP parser uses pairs of quotation marks to determine a string. Therefore, all strings must use the same single or double Quotation marks to define start and end. For example, the following string definition is illegal:

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"I am not a valid string since I have unmatching quote marks' 'Me neither!"

When defining a string, only one kind of quotation mark is considered as the delimiter, that is, single quotation mark or double quotation mark. So, if a string is represented by a double quote
, then only double quotes are parsed by the parser. This way, you can include any other character within the double-quoted string, even single-quoted
Number. The following quotation mark strings are legal:

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$s = "I am a 'single quote string' inside a double quote string"; $s = 'I am a "double quote string" inside a single quote string'; 　　当PHP遇到与串的开头相对应的引号时，便认为已经到了字符串尾部，于是： "Why doesn't "this" work?" 　　实际上被PHP语法分析器分成三个部分: "Why doesn't "——包含一个单引号的双引号串 this——多余的字符，分析器无法处理 " work?" ——普通字符串

$s = "I am a 'single quote string' inside a double quote string";

$s = 'I am a "double quote string" inside a single quote string';

When PHP encounters the quotation mark corresponding to the beginning of the string, it thinks that it has reached the end of the string, so:

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"Why doesn't "that" work?"

"Why doesn't "this" work?"

is actually divided into three parts by the PHP parser:

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'You'd better escape your apostrophes'

"Why doesn't " - a double quote string containing a single quote

this - extra characters, the parser cannot handle
"work?" - ordinary string

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$file = "c:windowssystem.ini"; echo $file; // 打印结果为： c:windowssystem.ini $file = "c:windowssystem.ini"; echo $file; // 打印结果为： c:windowssystem.ini

The above example attempts to include double quotes within a double quote string, and the parser considers the string to be terminated when it encounters the second double quote

Finished. To achieve the purpose of including quotation marks, the parser must ignore its original meaning when encountering ordinary quotation marks in the string. We use the quotation mark
Add a backslash in front of it to tell PHP that this quote is part of the string. The correct representation is like this:

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"Why doesn't "that" work?"

A common problem in English strings is the use of apostrophe ', because it is a single quote, and it is very common in English strings

(English possessive case). You have to be careful with these characters:

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$first_name = 'Charlie'; $last_name = 'Brown'; $full_name = $first_name . ' ' . $last_name;

'You'd better escape your apostrophes'

You can see that backslash has its special meaning in strings. When we need to include the backslash itself in the string, we need to include it in This symbol is preceded by an extra backslash. For example:

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$file = "c:windowssystem.ini"; echo $file; // The print result is: c:windowssystem.ini $file = "c:windowssystem.ini"; echo $file; // The print result is: c:windowssystem.ini

Another way to define strings that eliminates the trouble of special characters and makes it easier to quote longer texts. The string definition method The method starts with the 2. String connection Strings can be connected using the string concatenation character (.), such as:

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$first_name = 'Charlie'; $last_name = 'Brown'; $full_name = $first_name . ' ' . $last_name;

A common use is to create large blocks of HTML string code. The assignment symbol (=) and the connector (.) can be abbreviated and combined into the (.=) symbol
Number, such as:

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$html = '

';
$html .= '';
for ( $i=0 ; $i $square = $i * $i;
$html .= '';
}
$html .= '

number	square
' . $i . '	' . $square . '

';

$html = '';

$html .= '

';
for ( $i=0 ; $i $square = $i * $i;

$html .= '

';
} $html .= '

number	square
' . $i . '	' . $square . '

';

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$full_name = $first_name . ' ' . $last_name; $full_name = "$first_name $last_name";

3. Use variables in strings

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$foo = 2; echo "foo is $foo"; // 打印结果: foo is 2 echo 'foo is $foo'; // 打印结果: foo is $foo echo "foo is $foon"; // 打印结果: foo is 2 (同时换行) echo 'foo is $foon'; // 打印结果: foo is $foon

This feature allows you to join a large number of simple strings without using concatenation symbols. PHP allows us to include the word

directly in a double quoted string String variables, we can find that the processing results of the following two strings are the same.

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$full_name = $first_name . ' ' . $last_name; $full_name = "$first_name $last_name";

Single quote strings and double quote strings are processed differently in PHP. The content in double-quoted strings can be interpreted and replaced, while single-quoted

The contents of the number string are always considered ordinary characters. For example:

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echo "value = $foo"; echo "value = $a[$i]";

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$foo = 2;

echo "foo is $foo"; // Print result: foo is 2

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echo "value = $a[$i][$j]"; //我们希望打印二维数组$a的某个元素。

echo 'foo is $foo'; // Print result: foo is $foo echo "foo is $foon"; // Print result: foo is 2 (with newline) echo 'foo is $foon'; // Print result: foo is $foon

As you can see, even the backslash within a single-quote string loses its extended meaning (except for the insertion of a backslash and the insertion of a single quote '). Therefore, when you want to perform variable substitution and include escape sequences such as n (newline character) in a string, you should use double quotes Number. Single quote strings can be used anywhere else. It will be faster to use single quote strings in scripts because the PHP parser is sensitive to The processing method of single quotation mark string is relatively simple, while the processing of double quotation mark is more complicated because the string also needs to be parsed internally, so the processing speed is Slightly slower. Some problems may arise when referencing complex combinations of variables in strings. The following code will work normally:

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echo "value = $foo"; echo "value = $a[$i]";

But the following code cannot get the results we want:

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echo "value = $a[$i][$j]"; //We want to print an element of the two-dimensional array $a.

To avoid these potential problems in using strings, we usually separate complex variables from strings, like this:

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echo 'value = ' . $a[$i][$j];

echo 'value = ' . $a[$i][$j];

Another way is to enclose complex variables in curly braces, so that the parser can correctly identify them:

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echo "value = {$a[$i][$j]}" //打印二维数组$a的某个元素

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echo "value = {$a[$i][$j]}" //Print an element of the two-dimensional array $a

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$var = 3; echo "value = {$var}"; // 打印结果 "value = 3" echo "value = {$var}"; // 打印结果 "value = {3}"

In this way, a new problem arises. When we want to quote the curly brace character itself in a string, we need to remember to use the escape character:

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$var = 3; echo "value = {$var}"; // print result "value = 3" echo "value = {$var}"; // print result "value = {3}"

3. Slashes and SQL statements

Generating HTML code or SQL query statements is often encountered when writing PHP programs and is an interesting thing. Why do you say that?

Because this involves generating another type of code, you must carefully consider and follow the syntax and conventions required for writing this code

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select * from users where last_name = 'O'Keefe'

but.

Let’s look at an example. If you want to query the user whose name is “O’Keefe” in the database, the usual SQL statement is in the form

It goes like this:

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$last_name = "O'Keefe"; $sql = "select * from users where last_name = '" . addslashes($last_name) . "'";

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select * from users where last_name = 'O'Keefe'

Please note that the English possessive character (apostrophe) of the SQL statement needs to be escaped with a backslash. PHP provides some functions to handle this

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$sql = 'select * from users where last_name = '' . addslashes($last_name) . ''';

In this case, the purpose of the function AddSlashes($str) is to automatically insert the backslash escape character for the quote character in the string:

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$last_name = "O'Keefe"; $sql = "select * from users where last_name = '" . addslashes($last_name) . "'";

In this example, you also need to enclose the last_name string in single quotes (required by SQL syntax), since double is used here. A string of quotes, so there is no need to escape the pair of single quotes. The following statement is the equivalent of using a single quote string:

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$sql = 'select * from users where last_name = '' . addslashes($last_name) . ''';

Anytime you write a string into a database, you have to make sure that the quotes inside are properly escaped, which is a lot of PHP
Common mistakes made by beginners.

4. Double quotes and HTML

Unlike SQL statements, double quotes are often used to represent strings in standard HTML language (many browsers now have strong fault tolerance capabilities
Yes, it is allowed to use single quotes or even no quotes to represent strings in HTML), for example:

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$html = ''.$link.''; $html = "$link";

$html = ''.$link.'';

$html = "$link";

HTML language does not support backslash escaping, which will happen when we use hidden inputs of the form to transmit data Got it. The best way to set the value of hidden inputs is to use the htmlspecialchars() function to encode it. The following statements can be

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To transmit data that may contain double quotes normally:

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1. Quotes define strings. To achieve the purpose of including quotation marks, the parser must ignore its original meaning when encountering ordinary quotation marks in the string. We add a backslash in front of the quotation mark to tell PHP: This quotation mark is part of the string and is the correct representation. The method is this: single quote strings can be used anywhere else. The processing speed of using single quote strings in scripts will be faster, because the PHP parser processes single quote strings in a relatively simple way, while the processing of double quotes is due to the internal nature of the string. It also requires parsing, so it's more complex, so the processing speed is slightly slower.

This...double quotes are escaped, single quotes are not escaped

For example: /r/n is a newline, but if you write a file with single quotes, it will not be a newline, but a character. If you write a file with double quotes, it will be a newline.

Finally, check out what other websites have to say

” ” Fields enclosed in double quotes will be interpreted by the compiler and then output as HTML code.

‘ ‘ The words in the single quotes are not interpreted and are output directly.

It can be seen from the literal meaning that single quotes are faster than double quotes.

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$abc=’my name is tome’; echo $abc //结果是:my name is tom echo ‘$abc’ //结果是:$abc echo “$abc” //结果是:my name is tom

For example:

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$abc='my name is tome';

echo $abc //The result is: my name is tom

echo ‘$abc’ //The result is: $abc

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select * from abc_table where user_name=’abc’;

echo “$abc” //The result is: my name is tom

Especially when using MYSQL statements, the usage of double quotes and single quotes can be confusing to novices. Here, I will give an example to illustrate. Assume that constants are used in the query conditions, for example:

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select * from abc_table where user_name=’abc’;

SQL statement can be written as:

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SQLstr = “select * from abc_table where user _name= ‘abc’” ;

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$user_name = $_REQUEST['user_name']; //字符串变量

SQLstr = “select * from abc_table where user _name= ‘abc’” ;

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$user=array (”name”=> $_REQUEST['user_name‘,"age"=>$_REQUEST['age'];//数组变量

Assume that variables are used in the query conditions, for example:

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SQLstr = “select * from abc_table where user_name = ‘ ” . $user_name . ” ‘ “; SQLstr = “select * from abc_table where user_name = ‘ ” . $user["name"] . ” ‘ “;

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SQLstr=”select * from abc_table where user_name = ‘ abc ‘ ” ; SQLstr=”select * from abc_table where user_name =’ ” . $user _name . ” ‘ “; SQLstr=”select * from abc_table where user_name =’ ” . $user["name"] . ” ‘ “;

$user_name = $_REQUEST['user_name']; //String variable

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1：”select * from table where user_name = ‘ ” //固定SQL语句 2：$user //变量 3：” ‘ ” 1,2,3部分字符串之间用”.”

or

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$user=array ("name"=> $_REQUEST['user_name‘,"age"=>$_REQUEST['age'];//Array variable The SQL statement can be written as:

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SQLstr = “select * from abc_table where user_name = ‘ . $user_name . ” ‘ “; SQLstr = “select * from abc_table where user_name = ‘ ” . $user["name"] . ” ‘ “;

Compare:

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SQLstr=”select * from abc_table where user_name = ‘ abc ‘ ” ; SQLstr=”select * from abc_table where user_name =’ ” . $user _name . ” ‘ “; SQLstr=”select * from abc_table where user_name =’ ” . $user["name"] . ” ‘ “; SQLstr can be decomposed into the following 3 parts: http://www.bkjia.com/PHPjc/632232.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/632232.htmlTechArticleThe article uses a large number of implementations to introduce the usage and difference between single quotes and double quotes. Students who need to know more You can refer to this article carefully. In PHP, usually one character...