Detailed introduction to the difference between single quotes and double quotes in PHP_PHP Tutorial

The article introduces in detail the difference between single quotes and double quotes in PHP. Students who need to know more can refer to it.

1. Define string
In PHP, strings can be defined using single quotes or double quotes. However, the same single or double quotation marks must be used to define the string. For example, 'Hello' and 'Hello' are illegal string definitions. ​
When defining a string, only one type of quotation mark is considered a delimiter, either a single quotation mark or a double quotation mark. Thus, if a string begins with a double quote, only the double quote is parsed by the parser. This way, you can include any other character, even single quotes, within the double-quoted string. The following quotation mark strings are legal:

Php code

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

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

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

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

The string "Why doesn't "this" work?" will be divided into three paragraphs. If you want to express double quotes in this string, you can use the escape character "" (backslash) to become "Why doesn't "this" work?"

2. Single and double quotes in string variables

PHP allows us to directly include string variables in double-quoted strings. We can find that the processing results of the following two strings are the same.

$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 contents of a double-quoted string can be interpreted and replaced, while the contents of a single-quoted string are always considered ordinary characters. For example:

Php code

$foo = 2;

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

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

$foo = 2;

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

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 inserted backslash and the inserted single quote '). Therefore, when you want to perform variable substitution and include escape sequences such as n (newline) in a string, you should use double quotes. 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 also requires parsing inside the string. It is therefore more complex and therefore slightly slower to process. ​

Some problems may arise when referencing complex combinations of variables in strings. The following code will work fine:

Php code

echo "value = $foo";

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

echo "value = $foo";

echo "value = $a[$i]"; 　

The following code does not get the results we want:

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: echo 'value = ' . $a[$i][$j];/ / Use dot (.) to connect strings

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

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

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:

Php code

$var = 3;

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

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

$var = 3;

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

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

3. In SQL statement

This is a problem that is often encountered. The SQL statement inserted into the database uses single quotes to define the string. If you want to insert a string containing single quotes into the database, the SQL statement will go wrong.

For example: $sql="insert into userinfo (username,password) Values('O'Kefee','123456')" 　

At this time, one of the methods is to add the escape character backslash in the SQL statement,

That is:...Values('O'Kefee',... 　

Of course, you can also use the function addslashes(). The function of this function is to add escape characters,

That is: $s = addslashes("O'Kefee") ……Values('".$s."',…… 　

Another method is to set the magic-quotes option in php.ini. If this option is turned on, if there are single quotes in the information submitted through the form, escape characters will be automatically added. So there is no need to use other functions.

Supplement: This starts with the role of double quotes and single quotes: The fields in double quotes will be interpreted by the compiler and then output as HTML code, but the fields in single quotes do not need to be interpreted and will be output directly.

For example:

$abc='I love u';

echo $abc //The result is: I love u

echo '$abc' //The result is: $abc

echo "$abc" //The result is: I love u

So when assigning values ​​to SQL statements in the database, double quotes must be used. SQL="select a,b,c from..." However, there will be single quotes in the SQL statement to quote the field names

For example: select * from table where user='abc';

The SQL statement here can be written directly as SQL="select * from table where user='abc'"

But if it’s like this:

$user='abc';

SQL1="select * from table where user=' ".$user." ' "; Compare

SQL2="select * from table where user=' abc ' "

I added a little more space between the single quotes and double quotes, I hope you can see it more clearly.

That is, replace 'abc' with '".$user."' all within a single quote. Just split the entire SQL string. SQL1 can be broken down into the following 3 parts

1："select * from table where user=' "

2：$user

3：" ' "

Use . to connect strings

http://www.bkjia.com/PHPjc/629102.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/629102.htmlTechArticleThe article introduces in detail the difference between single quotes and double quotes in PHP. Students who need to know more about it Please refer to it. 1. Define a string In PHP, the definition of a string can be...