Prueba con esto:
Código PHP:
<html>
<head>
<script Language='JavaScript'>
function MSG() {
alert ('hola mundo!');
}
</script>
</head>
<body>
<form Id="Form1" method="POST">
<input type="submit" value="Enviar" name="B1">
</form>
<script for="Form1" event="onsubmit" Language='JavaScript'>
MSG();
</script>
</body>
</html>
O prueba con esto: Código PHP:
<html>
<head>
<script Language='JavaScript'>
</script>
</head>
<body>
<form Id="Form1" method="POST">
<input type="submit" value="Enviar" name="B1">
</form>
<script for="Form1" event="onsubmit" Language='JavaScript'>
alert ('hola mundo!');
</script>
</body>
</html>