check_ajax_referer

	/**
	 * Verifies the Ajax request to prevent processing requests external of the blog.
	 *
	 * @since 2.0.3
	 *
	 * @param int|string   $action    Action nonce.
	 * @param false|string $query_arg Optional. Key to check for the nonce in `$_REQUEST` (since 2.5). If false,
	 *                                `$_REQUEST` values will be evaluated for '_ajax_nonce', and '_wpnonce'
	 *                                (in that order). Default false.
	 * @param bool         $die       Optional. Whether to die early when the nonce cannot be verified.
	 *                                Default true.
	 * @return int|false 1 if the nonce is valid and generated between 0-12 hours ago,
	 *                   2 if the nonce is valid and generated between 12-24 hours ago.
	 *                   False if the nonce is invalid.
	 */
	function check_ajax_referer( $action = -1, $query_arg = false, $die = true ) {
		if ( -1 == $action ) {
			_doing_it_wrong( __FUNCTION__, __( 'You should specify a nonce action to be verified by using the first parameter.' ), '4.7' );
		}

		$nonce = '';

		if ( $query_arg && isset( $_REQUEST[ $query_arg ] ) ) {
			$nonce = $_REQUEST[ $query_arg ];
		} elseif ( isset( $_REQUEST['_ajax_nonce'] ) ) {
			$nonce = $_REQUEST['_ajax_nonce'];
		} elseif ( isset( $_REQUEST['_wpnonce'] ) ) {
			$nonce = $_REQUEST['_wpnonce'];
		}

		$result = wp_verify_nonce( $nonce, $action );

		/**
		 * Fires once the Ajax request has been validated or not.
		 *
		 * @since 2.1.0
		 *
		 * @param string    $action The Ajax nonce action.