Source

File: wp-admin/includes/post.php

function add_meta( $post_ID ) {
	$post_ID = (int) $post_ID;

	$metakeyselect = isset( $_POST['metakeyselect'] ) ? wp_unslash( trim( $_POST['metakeyselect'] ) ) : '';
	$metakeyinput  = isset( $_POST['metakeyinput'] ) ? wp_unslash( trim( $_POST['metakeyinput'] ) ) : '';
	$metavalue     = isset( $_POST['metavalue'] ) ? $_POST['metavalue'] : '';
	if ( is_string( $metavalue ) ) {
		$metavalue = trim( $metavalue );
	}

	if ( ( ( '#NONE#' !== $metakeyselect ) && ! empty( $metakeyselect ) ) || ! empty( $metakeyinput ) ) {
		/*
		 * We have a key/value pair. If both the select and the input
		 * for the key have data, the input takes precedence.
		 */
		if ( '#NONE#' !== $metakeyselect ) {
			$metakey = $metakeyselect;
		}

		if ( $metakeyinput ) {
			$metakey = $metakeyinput; // Default.
		}

		if ( is_protected_meta( $metakey, 'post' ) || ! current_user_can( 'add_post_meta', $post_ID, $metakey ) ) {
			return false;
		}

		$metakey = wp_slash( $metakey );

		return add_post_meta( $post_ID, $metakey, $metavalue );
	}

	return false;
}