Puoi usare jquery
codice:
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$('#password').keydown(function(event) {
if (event.keyCode == 13) {
this.form.submit();
return false;
}
});
});
</script>
</head>
<body>
<form type="get">
<input name="username" id="username" type="text" placeholder="username"/>
<input name="password" id="password" type="password" placeholder="password"/>
</form>
</body>
</html>
<?php
//Solo per controllo
echo print_r($_GET);
?>