autoload.php

<?php

spl_autoload_register(function ($class) {

    // project-specific namespace prefix
    $prefix = 'SocialPosts\\';

    // does the class use the namespace prefix?
    $len = strlen( $prefix );

    if ( strncmp( $prefix, $class, $len ) !== 0 ) {
        // no, move to the next registered autoloader
        return;
    }

    // get the relative class name
    $relative_class = substr( $class, $len );
    $first = strtok( $relative_class, '\\' );
    $base_dir = __DIR__ . '/classes/';

    // replace the namespace prefix with the base directory, replace namespace
    // separators with directory separators in the relative class name, append
    // with .php
    $file = $base_dir . str_replace( '\\', '/', $relative_class ) . '.php';

    // if the file exists, require it
    if ( file_exists($file) ) {
        require $file;
    }

});