How to prettify the code onkeyup or onmouseover event?

[MOSTAFAEZZAT]

I have a text area that I want display the code that in the textarea as a code.

I already used Prism classes but it doesn't work on the text area so I need to make the event or execute the function that when the user paste his code it displays as a code.

<pre class="language-php"> <code class="language-php">   <?php echo htmlspecialchars($str); ?>      </code></pre>
       <div class="language-php"> <textarea id="testcode" class="language-php" id="code"> </textarea></div>