List all PHP variables

Solution 1:

Use get_defined_vars and/or get_defined_constants

$arr = get_defined_vars();
print_r($arr);

Solution 2:

When debugging trying to find differences using a program such as WinMerge (freeware) to see what differences various arrays and variables have you'll want to ksort() otherwise you'll get lots of false negatives. It also helps to visually format using the HTML pre element...

<?php
$everything = get_defined_vars();
ksort($everything);

?>

Edit: had to come back to this and realized I had a better answer, $GLOBALS.

$a = print_r(var_dump($GLOBALS),1);
echo '<pre>';
echo htmlspecialchars($a);
echo '</pre>';

Edit 2: as mpag mentioned print_r() may be susceptible to running out of memory if the software you're working with uses a lot. Presuming there is no output or it's clearly truncated and you have access to the php.ini file you can adjust the memory use as so:

ini_set('memory_limit', '1024M');