PHP: simply get array values.

functions.php

<?php

/**
 * Get value from $_GET or defined $haystack.
 *
 * @since 1.0.0
 *
 * @param string $needle   Name of the searched key.
 * @param string $haystack Optional. Associative array. If false, $_GET is set to be the $haystack.
 * @param mixed  $default  Optional. Value returned if the searched key isn't found.
 *
 * @return string Value if found, $default otherwise.
 */
function beans_get( $needle, $haystack = false, $default = null ) {
	if ( $haystack === false ) {
		$haystack = $_GET;
	}

	$haystack = (array) $haystack;
	
	if ( isset( $haystack[$needle] ) ) {
		return $haystack[$needle];
	}
	
	return $default;
}