- preg_replace: documentation ( source)
<?php
$params = 'folder=BP+Portrait+Award+2014%2CBlah&format=jpg&public_id=test_dtestddddd_test_1407335836timetamp=1407335836';
$urlSafeParams = preg_replace('/(\?<=folder)*%2C/', ',', $params);
print $urlSafeParams;