WordPress at a glance
function is not described

spl_autoload_register() WP 4.6.0

Registers a function to be autoloaded.

No Hooks.

Return

Nothing (null).

Usage

spl_autoload_register( $autoload_function, $throw, $prepend );
$autoload_function(callable) (required)
The function to register.
$throw(true/false)
Whether the function should throw an exception if the function isn't callable.
Default: true
$prepend(true/false)
Whether the function should be prepended to the stack.
Default: false

Code of spl autoload register: wp-includes/spl-autoload-compat.php WP 5.2.1

<?php
function spl_autoload_register( $autoload_function, $throw = true, $prepend = false ) {
	if ( $throw && ! is_callable( $autoload_function ) ) {
		// String not translated to match PHP core.
		throw new Exception( 'Function not callable' );
	}

	global $_wp_spl_autoloaders;

	// Don't allow multiple registration.
	if ( in_array( $autoload_function, $_wp_spl_autoloaders ) ) {
		return;
	}

	if ( $prepend ) {
		array_unshift( $_wp_spl_autoloaders, $autoload_function );
	} else {
		$_wp_spl_autoloaders[] = $autoload_function;
	}
}