cp_object_relationship_exists( int $left_object_id, string $left_object_type, string $right_object_type, int $right_object_id )

function cp_object_relationship_exists( $left_object_id, $left_object_type, $right_object_type, $right_object_id ) {

	// Error if $left_object_id is not a positive integer.
	if ( filter_var( $left_object_id, FILTER_VALIDATE_INT ) === false ) {

		$item = 'string';
		if ( $left_object_id === 0 ) {
			$item = 0;
		} elseif ( is_object( $left_object_id ) ) {
			$item = 'object';
		} elseif ( is_array( $left_object_id ) ) {
			$item = 'array';
		}

		/* translators: %s: String, 0, object, or array. */
		$message = sprintf( __( 'cp_add_object_relationship() expects parameter 1 to a positive integer; %s given.' ), $item );

		return new WP_Error( 'left_object_id', $message );
	}

	// Error if $right_object_id is not a positive integer.
	if ( filter_var( $right_object_id, FILTER_VALIDATE_INT ) === false ) {

		$item = 'string';
		if ( $right_object_id === 0 ) {
			$item = 0;
		} elseif ( is_object( $right_object_id ) ) {
			$item = 'object';
		} elseif ( is_array( $right_object_id ) ) {
			$item = 'array';
		}

		/* translators: %s: String, 0, object, or array. */
		$message = sprintf( __( 'cp_add_object_relationship() expects parameter 4 to a positive integer; %s given.' ), $item );

		return new WP_Error( 'right_object_id', $message );
	}

	// Get array, and create list, of recognized relationship objects.
	$recognized_relationship_objects = cp_recognized_relationship_objects();
	$object_list = implode( ', ', $recognized_relationship_objects );

	// Error if $left_object_type is not a non-null string of an appropriate value.
	if ( ! in_array( $left_object_type, $recognized_relationship_objects ) ) {

		$item = $left_object_type;
		if ( is_int( $left_object_type ) ) {
			$item = 'integer';
		} elseif ( is_object( $left_object_type ) ) {
			$item = 'object';
		} elseif ( is_array( $left_object_type ) ) {
			$item = 'array';
		}

		/* translators: 1: List of recognized relationship objects, 2: Integer, object, or array. */
		$message = sprintf( __( 'cp_add_object_relationship() expects parameter 2 to be one of %s; %s given.' ), $object_list, $item );

		return new WP_Error( 'left_object_type', $message );
	}

	// Error if $right_object_type is not a non-null string of an appropriate value
	if ( ! in_array( $right_object_type, $recognized_relationship_objects ) ) {

		$item = $right_object_type;
		if ( is_int( $right_object_type ) ) {
			$item = 'integer';
		} elseif ( is_object( $right_object_type ) ) {
			$item = 'object';
		} elseif ( is_array( $right_object_type ) ) {
			$item = 'array';
		}

		/* translators: 1: List of recognized relationship objects, 2: Integer, object, or array. */
		$message = sprintf( __( 'cp_add_object_relationship() expects parameter 3 to be one of %s; %s given.' ), $object_list, $item );

		return new WP_Error( 'right_object_type', $message );
	}

	// Good to go, so query database table.
	global $wpdb;
	$table_name = $wpdb->prefix . 'object_relationships';
	$relationship_id = 0;

	$relationship_array = array(
		'left_object_id'    => $left_object_id,
		'left_object_type'  => $left_object_type,
		'right_object_type' => $right_object_type,
		'right_object_id'   => $right_object_id,
	);

	// Check if this relationship already exists.
	$sql1 = $wpdb->prepare( "SELECT relationship_id FROM $table_name WHERE left_object_id = %d AND left_object_type = %s AND right_object_type = %s AND right_object_id = %d", $left_object_id, $left_object_type, $right_object_type, $right_object_id );

	// If so, return the relationship ID as an integer.
	$row = $wpdb->get_row( $sql1 );
	if ( is_object( $row ) ) {
		$relationship_id = (int) $row->relationship_id;
	}

	if ( empty( $relationship_id ) ) {

		// Also query database table right to left if no match so far.
		$sql2 = $wpdb->prepare( "SELECT relationship_id FROM $table_name WHERE right_object_id = %d AND right_object_type = %s AND left_object_type = %s AND left_object_id = %d", $left_object_id, $left_object_type, $right_object_type, $right_object_id );

		// If this relationship exists, return the relationship ID as an integer.
		$row = $wpdb->get_row( $sql2 );
		if ( is_object( $row ) ) {
			$relationship_id = (int) $row->relationship_id;
		}
	}

	// Hook when search for pre-existing relationship completed.
	do_action( 'existing_object_relationship', $relationship_id, $left_object_id, $left_object_type, $right_object_type, $right_object_id );

	// Return relationship ID (which will be 0 if none exists).
	return $relationship_id;
}