core-bundle/src/Resources/contao/library/Contao/Input.php
public static function setGet($strKey, $varValue, $blnAddUnused=false)
	{
		// Convert special characters (see #7829)
		$strKey = str_replace(array(' ', '.', '['), '_', $strKey);

		$strKey = static::cleanKey($strKey);

		unset(static::$arrCache['getEncoded'][$strKey]);
		unset(static::$arrCache['getDecoded'][$strKey]);

		if ($varValue === null)
		{
			unset($_GET[$strKey]);
		}
		else
		{
			$_GET[$strKey] = $varValue;

			if ($blnAddUnused)
			{
				static::setUnusedGet($strKey, $varValue); // see #4277
			}
		}
	}
public static function setGet($strKey, $varValue, $blnAddUnused=false)
	{
		// Convert special characters (see #7829)
		$strKey = str_replace(array(' ', '.', '['), '_', $strKey);

		$strKey = static::cleanKey($strKey);

		unset(static::$arrCache['getEncoded'][$strKey]);
		unset(static::$arrCache['getDecoded'][$strKey]);

		if ($varValue === null)
		{
			unset($_GET[$strKey]);
		}
		else
		{
			$_GET[$strKey] = $varValue;

			if ($blnAddUnused)
			{
				static::setUnusedGet($strKey, $varValue); // see #4277
			}
		}
	}
Set a $_GET variable

@param string  $strKey       The variable name
@param mixed   $varValue     The variable value
@param boolean $blnAddUnused If true, the value usage will be checked