- implode: documentation ( source)
<?php
$categories = array('ammunition'=>'Ammunition', 'brass'=>'Brass Casings', 'bullets'=>'Projectiles', 'guns'=>'Firearms', 'magazines'=>'Magazines', 'powder'=>'Powder', 'primers'=>'Primers', 'reloading_misc'=>'Reloading Equipment');
$categories = '"'.implode('", "', $categories).'"';
echo $categories.'<br />';
echo $categories["ammunition"];
?>