$re = '/,(?!(?:[^"]*"[^"]*")*[^"]*$)/';
$str = '"COURSE",247,"28/4/2016 12:53 Europe/Brussels",1,"Verschil tussen merk, product en leveranciersverantwoordelijke NL","Active Enro"';
$subst = "";
$result = preg_replace($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