First, is your form set up right? In order to get data from $_POST you have to have:
<form action="" method="post">
...your form goes here...
<input type="submit" value="Whaver you want the button to say goes here">
</form>
The 'action' attribute is the page that will load when the user hits the submit button, if you define it to be empty ("") then it will just reload this same page.
Then if you have your form element like so:
<textarea name="oldvideo" id="oldvideo"><?php echo $embedCode; ?></textarea>
...then when the user types something into the textarea and hits submit it will be saved to $_POST['oldvideo'].
Whatever you have saved to the variable $embedCode will just be sitting in the textarea as it's default value until someone types in anything else. If you don't need them to have a default value then you could leave that out.
Lastly, you should make sure your code is setup like so...
if( isset($_POST['oldvideo'] ) {
$embedCode = $_POST['oldvideo'];
...the rest of your code that makes use of the user-submitted value...
} else {
$embedCode = "The Default Value for Embed Code Goes Here";
}
If you try to use $_POST['oldvideo'] without putting inside an if(isset()) block, then you'll get an error when you try to load the page.
Hope this helps, but it would also be easier to help you if you gave us a little more detail about your problem!