core-bundle/src/Resources/contao/library/Contao/Input.php
public static function get($strKey, $blnDecodeEntities=false, $blnKeepUnused=false)
	{
		if (!isset($_GET[$strKey]))
		{
			return null;
		}

		$strCacheKey = $blnDecodeEntities ? 'getDecoded' : 'getEncoded';

		if (!isset(static::$arrCache[$strCacheKey][$strKey]))
		{
			$varValue = $_GET[$strKey];

			$varValue = static::decodeEntities($varValue);
			$varValue = static::xssClean($varValue, true);
			$varValue = static::stripTags($varValue);

			if (!$blnDecodeEntities)
			{
				$varValue = static::encodeSpecialChars($varValue);
			}

			$varValue = static::encodeInsertTags($varValue);

			static::$arrCache[$strCacheKey][$strKey] = $varValue;
		}

		// Mark the parameter as used (see #4277)
		if (!$blnKeepUnused)
		{
			unset(static::$arrUnusedGet[$strKey]);
		}

		return static::$arrCache[$strCacheKey][$strKey];
	}
public static function get($strKey, $blnDecodeEntities=false, $blnKeepUnused=false)
	{
		if (!isset($_GET[$strKey]))
		{
			return null;
		}

		$strCacheKey = $blnDecodeEntities ? 'getDecoded' : 'getEncoded';

		if (!isset(static::$arrCache[$strCacheKey][$strKey]))
		{
			$varValue = $_GET[$strKey];

			$varValue = static::decodeEntities($varValue);
			$varValue = static::xssClean($varValue, true);
			$varValue = static::stripTags($varValue);

			if (!$blnDecodeEntities)
			{
				$varValue = static::encodeSpecialChars($varValue);
			}

			$varValue = static::encodeInsertTags($varValue);

			static::$arrCache[$strCacheKey][$strKey] = $varValue;
		}

		// Mark the parameter as used (see #4277)
		if (!$blnKeepUnused)
		{
			unset(static::$arrUnusedGet[$strKey]);
		}

		return static::$arrCache[$strCacheKey][$strKey];
	}
Return a $_GET variable

@param string  $strKey            The variable name
@param boolean $blnDecodeEntities If true, all entities will be decoded
@param boolean $blnKeepUnused     If true, the parameter will not be marked as used (see #4277)

@return mixed The cleaned variable value