WordPress at a glance
function is not described

WP_Query::__isset() WP 4.0.0

Make private properties checkable for backward compatibility.

This is a method of the class: WP_Query

No Hooks.

Return

true/false. Whether the property is set.

Usage

$WP_Query = new WP_Query();
$var = $WP_Query->__isset( $name );
$name(string) (required)
Property to check if set.

Changelog

Since 4.0.0 Introduced.

Code of WP Query:: isset: wp-includes/class-wp-query.php WP 5.2.3

<?php
public function __isset( $name ) {
	if ( in_array( $name, $this->compat_fields ) ) {
		return isset( $this->$name );
	}
}