PHP single and double quotes differences

PHP single double quotation mark difference

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

However, you must use the same single or double quotation mark to define a string, such as: 'Hello World" and "Hello World' an illegal string definition.

What's the difference between a single quote and a double quote?

PHP allows us to include it directly in a double quote string 字串变量 .

The contents of a single quote string are always considered ordinary characters, so the content in single quotes is not escaped more efficiently.

Like what:

$str = ' Hello '; echo "str is $STR"; Operation Result: STR is Helloecho ' str is $str '; Operation Result: STR is $STR

In PHP, the variables ($var) and special characters (\ r \ n) in double quotes are escaped, and the content in single quotes is not escaped (so it is more efficient).
Use on the words,
I used to like to write in a SQL string like this $sql = "SELECT * FROM table WHERE id = $id", so that the $id inside can be escaped, single quotes will not work.

There is no difference between single and double quotes in JavaScript, as long as paired use is OK.
I use single quotes in JavaScript mostly because JavaScript deals more with HTML, and you don't need to escape the quotes of attributes in HTML when outputting HTML fragments.
In short, see the actual situation to use, how convenient how to use.

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