使用tokenizer.

$fileStr = file_get_contents('testdemo.php');
$newStr  = '';

$tokens = token_get_all($fileStr);

foreach ($tokens as $token) {
    $commentTokens = array(T_COMMENT);

    if (defined('T_DOC_COMMENT'))
        $commentTokens[] = T_DOC_COMMENT;
    if (defined('T_ML_COMMENT'))
        $commentTokens[] = T_ML_COMMENT;

    if (is_array($token)) {
        if (in_array($token[0], $commentTokens))
            continue;

        $token = $token[1];
    }

    $newStr .= $token;
}

file_put_contents('testdemo.done.php',$newStr);