Probando abrir e imprimir un html:
ppp.htm Código PHP:
<html>
<head>
<script language="JavaScript">
function openToPrint(theURL,winName,features){
var oWnd = open(theURL,winName,features);
oWnd.print();
}
</script>
</head>
<body>
<form>
<input type="button" name="cmdImprimir" value="Imprimir" onclick="openToPrint('pppp.htm','','')">
</form>
</body>
</html>
pppp.htm Código PHP:
<html>
<head>
</head>
<body>
<form>
Hola
</form>
</body>
</html>
Pruébalo, a mi me funciona.
Saludos.