PHP and backslashes in strings

Question

Can anyone tell me what is happening here?

<?php
// true
var_dump('\\ ' === '\ ');

// false
var_dump('\\\\ ' === '\\ ');

// true
var_dump('\\\\ ' === '\\\ ');

Answer 1

\ inside a string literal introduces several types of escape sequences, \\ is the escape sequence for a literal "\". But, \s that don't resolve to an escape sequence are also taken as literal "\".

Therefor, '\\ ' stands for the string "\ ", '\\\\ ' stands for the string "\\ ", just as '\\\ '. Try:

echo '\\\\ ';   -> \\ 

See http://php.net/manual/en/language.types.string.php#language.types.string.syntax.single.

Answer 2

In single quoted strings, no escape sequences are interpolated. A backslash is only an escape character if it immediately precedes a single quote, or a backslash.

So:

var_dump('\\ '); // String (2) "\ "
var_dump('\ '); // String (2) "\ "
// They do match

var_dump('\\\\ '); // String (3) "\\ "
var_dump('\\ '); // String (2) "\ "
// They don't match

var_dump('\\\\ '); // String (3) "\\ "
var_dump('\\\ '); // String (3) "\\ "
// They do match

This is expected and documented behaviour, although it can be difficult to wrap you head around on the face of it.