How to debug in php

debug.php

<?php

private function log($args)
{
    $str = '';
    foreach ($args as $arg) {
        if (is_string($arg)) {
            $str .= $arg;
        } else {
            ob_start();
            var_dump($arg);
            $str .= ob_get_clean();
        }
    }
    $str .= "\n";
    file_put_contents('/some-mount-point-shared-with-host/log.log', $str, FILE_APPEND);
}