Remove a certain variable from a query string

Question

How do I remove a certain variable from a query string? Say I have a query string

$query_string = "first=val1&amp;second=val2&amp;third=val3";

function removevar($var, $query_string) {
    return preg_replace("/(".$var."=[^&]*(&amp;))/i","",$query_string);
}

echo removevar("first",$query_string); // ok
echo removevar("second",$query_string); // ok
echo removevar("third",$query_string); // doesn't change the string because third doesn't have a trailing &amp;

How can this be fixed so that it removes variables from a query string in a robust way? Probably someone already has a function that does this along with special cases in more complex strings.

So I'd have to match either & or end of the string ($) but I don't know how to turn that into regex.

why you want to do this, you already have the $_GET array, you can just take the parameters you need and if you really want to remove one, just rebuilt the string using the array without this one... — JohnJohnGa
– JohnJohnGa, Commented Apr 3, 2011 at 20:00
To make it more robust, I would ensure that the $var is "regex safe" by running it through preg_quote($var,'/') first. — ridgerunner
– ridgerunner, Commented Apr 3, 2011 at 20:10

buschtoens · Accepted Answer · 2011-04-03 20:00:35Z

3

$query_string = "first=val1&amp;second=val2&amp;third=val3";

function removevar($var, $query_string) {
    return preg_replace("/(".$var."=[^&]*(&amp;|$))/i","",$query_string);
}

echo removevar("first",$query_string); // ok
echo removevar("second",$query_string); // ok
echo removevar("third",$query_string); // ok

This should do the trick.

answered Apr 3, 2011 at 20:00

buschtoens

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Gumbo · Accepted Answer · 2011-04-03 20:01:45Z

3

You don’t necessarily need regular expressions for this as PHP does have functions that can parse and build query strings (parse_str and http_build_query respectively):

function removevar($var, $query_string) {
    parse_str($query_string, $args);
    unset($args[$var]);
    return http_build_query($args);
}

Note that you need to decode the HTML character references before using these functions.

answered Apr 3, 2011 at 20:01

Gumbo

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1 Comment

A-OK Over a year ago

Thanks to everyone who suggested the parse_str and http_build_query. Seems like PHP has functions for everything. I will be using that in the future but for reasons I can't go into I needed the regex for this in any case.

Wrikken · Accepted Answer · 2011-04-03 20:01:42Z

2

You will probably have more luck using:

html_entity_decode to get the 'normal' query-string.
parse_str to get the query string into an array.
unset the desired key in that array.
Use http_build_query to rebuild the string.
Call htmlspecialchars on it to get the & back to &.

Less concise than the regex route, but a lot less error-prone.

answered Apr 3, 2011 at 20:01

Wrikken

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Comments

Knowledge Craving · Accepted Answer · 2011-04-03 20:32:21Z

2

<?php
$query_string = "first=val1&amp;second=val2&amp;third=val3";
parse_str($query_string, $output);

function removevar($var, $output_array) {
  if (in_array($var, $output_array)) {
    unset($output_array[$var]);
  }

  return http_build_query($output_array, '', '&amp;');
}

echo removevar("first", $output);
echo removevar("third", $output);
?>

Normally for codes like this, which involves a query string, it is always best to go for the in-built PHP functions rather than for the regex syntax / formula. That is what I've done, and the main parts of the code include the following PHP in-built functions:-

parse_str Function
in_array Function
http_build_query Function

Hope it helps.

edited Apr 3, 2011 at 20:32

answered Apr 3, 2011 at 20:03

Knowledge Craving

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Dejan Marjanović · Accepted Answer · 2011-04-03 20:01:21Z

1

function removevar($var, $query_string) {
    $query_string = preg_replace("#$var=([^&]+)#is", "", $query_string);
    return trim($query_string, "&amp;");
}

answered Apr 3, 2011 at 20:01

Dejan Marjanović

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KingCrunch · Accepted Answer · 2011-04-03 20:04:23Z

1

There are functions, that can handle query strings directly.

function removevar ($var, $query_string) {
  $array = array();
  parse_str(html_entity_decode($query_string), $array);
  unset($array[$var]);
  return html_entities(http_build_query($array);
}

answered Apr 3, 2011 at 20:04

KingCrunch

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