Source

File: wp-includes/functions.php

function _scalar_wp_die_handler( $message = '', $title = '', $args = array() ) {
	list( $message, $title, $parsed_args ) = _wp_die_process_input( $message, $title, $args );

	if ( $parsed_args['exit'] ) {
		if ( is_scalar( $message ) ) {
			die( (string) $message );
		}
		die();
	}

	if ( is_scalar( $message ) ) {
		echo (string) $message;
	}
}