How to use spl_autoload_register() function to load all classes from plugin.

autoload-main.php

<?php 
/**
 * Plugin Name: Class Autoloader
 * Author: Govind
 * Author URI: http://emgk.github.io
 * Email: [email protected]
 *
 * Suppose, In our WP plugin we have three classes inside 'classes' folder. 
 * to load them without long lines of require_once statements, then in that
 * case we have to use spl_autoload_register() function. 
 * 
 * Plugin folder structure
 * 
 *   - Plugin Root Folder			
 * 	+-- classes				[DIR]
 *     		+-- class-wpauto-first.php	[FILE]
 *       	+-- class-wpauto-second.php	[FILE]
 *        	+-- class-wpauto-third.php	[FILE]
 *      +-- autoloader-main.php 		[FILE]
 */

// Registering our function into spl_autoload_register().
spl_autoload_register('autoloadclass');

// Define our function to load our classes.
function autoloadclass($classname){

	/*
	* As our class name is like WPAUTO_First,WPAUTO_Second,WPAUTO_Third, 
	* we've added 'WPAUTO' as class prefix to distinct them from other classes.
	* So that we can identify our classes.
	*	
	*/

	// exploding classname using '_' to get the prefix of our class.
	$classprefix = explode('_',strtolower($classname));

	// verify if class name containing 'wpauto' as prefix.
	if( 'wpauto' !== $classprefix[0] )
		return;

	// prepare the path of class folder.
	$dir = plugin_dir_path( __FILE__ ) . 'classes/';

	// generate file name from Class name. 
	// 'AUTO_First' will be 'class-auto-first.php'
	$classfile = 'class-' . str_replace('_', '-', strtolower( $classname ) ) . '.php';

	// checking if file exists or not.
	if ( file_exists( $classfile ) )
		include $classfile;

} 

?>

class-auto-first.php

<?php 

class AUTO_First{
	
	public function printdata(){
		echo 'I am inside first class!';
	}
}

class-auto-second.php

<?php 

class AUTO_Second{
	
	public function printdata(){
		echo 'I am inside Second class!';
	}
}

class-auto-third.php

<?php 

class AUTO_Third{
	
	public function printdata(){
		echo 'I am inside third class!';
	}
}