NULL is equal ''

emptiness_and_arrays.php

<?php
// http://www.phpwtf.org/

$a = array();
$a[''] = 'string index';
$a[NULL] = 'NULL index';
$a[] = 'add index';
var_dump($a);
// NULL is equal ''

$b = array();
$b[''] = 'string index';
$b[] = 'add index';
var_dump($b);

$a = 0;
$b = 'x';
var_dump(FALSE == $a);
var_dump($a == $b);
var_dump($b == TRUE);\
// compare integer and other type, convert integer first
// compare string and other type, convert string first

var_dump(NULL - 1);
// output
// int(-1)

// If you compare two numerical strings, they are compared as integers.
var_dump("15" == "0xF");
var_dump("15" === "0xF");
// bool(true)
// bool(false)

// So the restriction that the variable name can not start with a number
// is only enforced by the parser and even then it's fairly easy to circumvent. 
$a = 1;
$$a = 'foo';
print ${1};

?>