PHP single quotes and double quotes usage

Many codes sometimes use single quotes or double quotes to contain string content. In fact, a simple summary is that variables in double quotes can be parsed, and single quotes are absolute strings. . Let me introduce it to you in detail below.

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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 kind of quotation mark is considered as the delimiter, that is, single quotation mark or 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:

$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 quotation marks in string variables

PHP allows us to directly include string variables in double quotation mark strings. We It can be found that the processing results of the two strings below 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:

$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 insertion of 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. 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.

When referencing complex variable combinations in strings, some problems may occur. The following code will work normally:

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][$j]"; //我们希望打印二维数组$a的某个元素。 　　

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

echo 'value = ' . $a[$i][$j];//字符串的连接用点(.)

Another way is to separate complex variables Enclosed in curly braces, the syntax analyzer will recognize it correctly:

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

3. In the SQL statement

This is often the case The problem encountered is that the SQL statement inserted into the database uses single quotes to define the string. If a string containing single quotes is inserted 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 processing 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. Therefore, there is no need to use other Function.

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 are not required. Explanation, 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

Therefore, when assigning values ​​to SQL statements in the database, they must also be used in double quotes SQL="select a,b,c from .. ." But there will be single quotes in the SQL statement to quote the field name

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 like the following

$user='abc'; 
SQL1="select * from table where user=' ".$user." ' ";对比一下 
SQL2="select * from table where user=' abc ' "

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

That means replacing 'abc' with '".$user."' all within a single quote. It just splits the entire SQL string. SQL1 can be decomposed into the following three parts

1: "select * from table where user=' "

2: $user

3: " ' "

Use . to connect strings, so that Understand.

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