$re = '/,(?![^()]*\))/im';
$str = 'first_name=James(Mr), cust_id=98503(ZZW_LG,WGE,zonaire), StartDate=2015-05-20, EndDate=2015-05-20, performance=best';
$subst = "\n";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php