HTML Definir valor de textarea
// To set the textarea value, you must insert the TEXT into the element.
<textarea>VALUE</textarea>
// javascript
var textarea = `<textarea>${value}</textarea>`;
document.getElementById('my_textarea_id').textContent = 'foo'
// php
$value = 'foo';
$textarea = "<textarea>$value</textarea>";
echo $textarea;
NotDamian