php variables, strings and arrays: associative array returns first letter of string

Question

I understand that, with a sting assigned to a variable, individual characters can be expressed by using the variable as an indexed array, but why does the code below, using an associative array, not just die with missing required? Why does 'isset' not throw FALSE on an array key that definitely doesn't exist?

unset($a); 

$a = 'TESTSTRING'; 

if(!isset($a['anystring'])){ 
    die('MISSING REQUIRED'); 
}else{ 
    var_dump($a['anystring']);
} 

The above example will output:

string(1) "T"

EDIT:

As indicated by Jelle Keiser, this is probably the safer thing to do:

if(!array_key_exists('required',$_POST)){ 
    die('MISSING REQUIRED'); 
}else{ 
    echo $_POST['required']; 
} 

Answer 1

What PHP is doing is using your string as a numeric index. In this case, 'anystring' is the equivalent of 0. You can test this by doing

<?php
echo (int)'anystring';
// 0
var_dump('anystring' == 0);
// bool(true)

PHP does a lot of type juggling, which can lead to "interesting" results.

Answer 2

$a is a string not an associative array.

If you want to access it that way you have to do something like this.

$a['anystring'] = 'TESTSTRING';