PHPSpellCheck can be set to spell check multiple fields in several ways - using the Fields property of either the SpellCheckButton or SpellAsYouType Class.
$mySpell->Fields = "ALL";
$mySpell->Fields = "MyField1,MyHTMLElement2,MyIFrame3,MyTextInput4";
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$mySpell->Fields = "TEXTAREAS";
$mySpell->Fields = "TEXTINPUTS";
$mySpell->Fields = "EDITORS";