Well? Do you know the name of your textbox? If you did, say like this:
<input name="formula" type="text" value="3+6+7+5">
Then in your form handling php script:
$parts = explode('+', $_POST['formula']);
If you need to someday split on other operands, like - * / etc, then you probably should check out preg_split() at the php site.