Let's talk about the detailed explanation of the difference between single and double quotation marks in PHP, the detailed explanation of the difference between double quotation marks in PHP_PHP Tutorial

Let’s talk about the detailed explanation of the difference between single and double quotes in PHP, and the detailed explanation of the difference between double quotes in PHP

In PHP, the definition of a string can use English single quotes ' ', or you can use English double quotation marks "".

But the same type of single or double quotes must be used to define the string. For example, 'Hello World' and 'Hello World' are illegal string definitions.

What is the difference between single quotes and double quotes? Let’s learn about it through this article.

1. Define string 　

In PHP, the definition of a string can use 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"; // 打印结果: foo is 2 
echo 'foo is $foo'; // 打印结果: foo is $foo 
echo "foo is $foo\n"; // 打印结果: foo is 2 (同时换行) 
echo 'foo is $foo\n'; // 打印结果: foo is $foo\n 
$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 $foo\n"; // 打印结果: foo is 2 (同时换行) 
echo 'foo is $foo\n'; // 打印结果: foo is $foo\n 　

As you can see, even the backslash within a single quote string loses its extended meaning (except for the insertion of backslash \ and the insertion of single quote '). Therefore, you should use double quotes when you want to perform variable substitution and include escape sequences such as n (newline) in a string. 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 needs to be parsed 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]"; 

But the following code cannot 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][$j]}" //打印二维数组$a的某个元素

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}"; // 打印结果 "value = 3" 
echo "value = \{$var}"; // 打印结果 "value = {3}" 
$var = 3; 
echo "value = {$var}"; // 打印结果 "value = 3" 
echo "value = \{$var}"; // 打印结果 "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: Fields in double quotes will be interpreted by the compiler and then output as HTML code, but fields in single quotes are not required Interpretation and output directly.

For example:

$abc='I love u'; 
echo $abc //结果是:I love u 
echo '$abc' //结果是:$abc 
echo "$abc" //结果是: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." ' ";对比一下 
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, so you can understand.

The above is a detailed explanation of the difference between single and double quotes in PHP introduced by the editor. I hope it will be helpful to you. If you want to know more, please pay attention to the Bangkejia website!

http://www.bkjia.com/PHPjc/1135007.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/1135007.htmlTechArticle Let’s talk about the detailed explanation of the difference between single and double quotation marks in PHP. The detailed explanation of the difference between double quotation marks in php. In PHP, the definition of strings You can use English single quotes ' ' or English double quotes " ". But you must...