Ensure that an associative array contains values for all required keys and only those keys. (Basically the PHP5 equivalent of Textpattern's lAtts() function. It's one of those things I'm sick of having to figure out every time I need it.)

latts.php

<?php

$defaults=array('a'=>1, 'b'=>2, 'c'=>3);
$input=array('a'=> 'boo', 'c'=>4, 'd'=>5, 'e'=>6);

$output = array_intersect_key($input+$defaults, $defaults);

//Might want to provide a warning if $unused isn't empty.
$unused = array_diff_key($input, $defaults);