PHPSpellCheck can be set to spell check the contents any Frame or iFrame using the Fields property of the SpellCheckButton Class. Inline spellchecking cannot be applied to frames.
Note - that it is not possible to spell-check an iframe that points to a remote domain due to browser XSS security.
$mySpell->Fields = "myIFrameId";
Setting the field to the ID attribute of your iframe tag is the most specific way to target an Iframe.
$mySpell->Fields = "IFRAME:0";
In some instances - iFrames may not have Id attributes. In this case - you can acess them by their index in the Frames collection.