Description:
------------
When a single quote is backslashed within a single quote (e.g. '\''), the result is a one-character string consisting of the single quote. However, when the same is done within a *double*-quoted string, the result is a two character string (\'). This behavior is unexpected, since in every other way, double quoted strings are *more* interpretive than single quoted strings. Furthermore, the manual (http://php.net/manual/en/language.types.string.php#language.types.string.syntax.single) says

> To specify a literal single quote, escape it with a backslash (\). To specify a literal backslash, double it (\\).
> ...
> If the string is enclosed in double-quotes ("), PHP will interpret **more** [emphasis mine] escape sequences for special characters: 
> ...
> As in single quoted strings, escaping any other character will result in the backslash being printed too. Before PHP 5.1.1, the backslash in \{$var} had not been printed. 

In my mind, this is a bug - namely, the double quoted version should also accept escaped single quotes, since this true in most other languages:

JavaScript
    "\'" === '\'' // ==> true
Python
    '\'' == "\'" // ==> true
Ruby
    print "\'" == '\'' // ==> true
Perl
    #!/usr/bin/perl
    print "\'" == '\''; ==> 1 (true)
bash (no, but printf does)
    echo "\'" # ==> \'
    printf "\'" # ==> '
C/C++
    #include <stdio.h>
    int main (void) {
      printf("\'"); // ==> '
      return 0;
    }
Java
    public class Temp {
      public static void main (String[] args) {
        System.out.println("\'"); // ==> '
      }
    }

However, if this is a bug that has existed for a long time (and not intentional), then changing it might break backward compatibility. In any event, the documentation should be made *very* clear about this issue. In my case, it was causing inappropriately quoted sql, e.g. allowing an attack. I'm sure that many other people who assume the behavior to be the same as in other languages might make the same mistake.

Test script:
---------------
<?php

echo "\'";

// Expected: '
// Actual: \'