Added "set:" preffix to check the parameter was set

base.php

@@ -100,6 +100,9 @@ function arg($arg, $default=null, $validator=null) {
 	} else if (strpos($arg,'b64:') === 0) {
 		$type = 'base64';
 		$arg = substr($arg, 4);
+	} else if (strpos($arg,'set:') === 0) {
+		$type = 'exists';
+		$arg = substr($arg, 4);
 	}
 
 	$ps = explode('.', $arg);
@@ -116,6 +119,9 @@ function arg($arg, $default=null, $validator=null) {
 	if ($arr === null)
 		$arr = $ps[0];
 	eval('$value = isset($'.$arr.'["'.$ps[1].'"]) ? $'.$arr.'["'.$ps[1].'"] : null;');
+
+	if ($type === 'exists')
+		return $value !== null;
 	if ($value === null)
 		return !$require ? $default : apiresult(['error' => '<'.$arg.'> parameter must be specified'], 400);