- print_r: documentation ( source)
- preg_split: documentation ( source)
<?php
$str = 'a\\\\,b,c\\,d';
print_r(preg_split('/(?<![^\\\\][\\\\]),/', $str));
print_r(preg_split('/(?<!(?<!\\\\)\\\\),/', $str));
print_r(preg_split('/(?<![^\\\\])(?<!\\\\),/', $str));