删除部分字符串

<?php
 $test = 'marke=' . $_GET["marke"] . '&farbe=' . $_GET["farbe"]. '&groesse=' . $_GET["groesse"];
?>

 <?php
echo $test;
?>

echo would be: marke=diesel&farbe=blau&groesse=xl

How can i remove &farbe=blau from $test ?

Use parse_str and http_build_query to reliably parse and create query-strings (which also takes care of nasty url-encoding issues):

// You could probably have created $test with:
// $test = http_build_query($_GET);
parse_str($test,$values);
unset($values['farbe']);
$test = http_build_query($values);

Have you tried using a regular expression with preg_replace()? Off the top of my head, try something like:

preg_replace('/&farbe=.+?&/','',$test);