PSR4 Autoloader

autoload.php

<?php

/**
 * PSR-4 Autoloader.
 */
spl_autoload_register( function ( $class ) {

	// validate namespace
	if (
		strncmp(
			$namespace = 'Vendor\\Package',#namespace
			$class,
			$namespace_length = strlen( $namespace )
		) !== 0
	) {
		return;
	};

	// try to load the file
	if ( file_exists(
		$file = join(
	        DIRECTORY_SEPARATOR,
	        array(
		        __DIR__,
		        'src',#sources directory name
		        str_replace( '\\', DIRECTORY_SEPARATOR, trim( substr( $class, $namespace_length ), '\\' ) )
	        )
        ) . '.php'
	) ) {
		include $file;
	}
} );

PSR-4 autoloader for your packages that uses DIRECTORY_SEPARATOR constant. Just make sure to update the values marked with comments:

• #namespace
• #sources directory name