在CodeMirror TextArea上设置默认语言

如何设置CodeMirror TextArea在不回显标记的情况下理解PHP？

文本区域

<textarea id="code" name="code">
  <?php
    echo '<?php';
  ?>
</textarea>

JS脚本

var editor = CodeMirror.fromTextArea(document.getElementById("code"), {
        lineNumbers: false,
        matchBrackets: true,
        mode: "application/x-httpd-php",
        indentUnit: 4,
        indentWithTabs: true,
        enterMode: "keep",
        tabMode: "shift"
    });