<?php

$data = <<<DATA
"foo"
'foo'
abc "foo"
foo "bar", "baz"
abc 'foo "bar" baz'
DATA;

$regex = "~
		'[^']*'(*SKIP)(*FAIL) # match everything between single quotes and fail 
		|                     # or
		\"([^\"]*)\"		  # match double quotes
		~x";

$data = preg_replace($regex, "'$1'", $data);

echo $data;
?>