<?php
if (isset($var))
{
  //execute PHP script required (Update, insert etc)
}

else
{
?>
<script language="javascript">
//Copied from martin.f2o.org/javascript/form-validation
//Validates a number
function validateNumber(field, msg, min, max) {
    if (!min) { min = 0 }
    if (!max) { max = 255 }

    if ( (parseInt(field.value) != field.value) ||
             field.value.length < min ||
             field.value.length > max) {
        alert(msg);
        field.focus();
        field.select();
        return false;
    }

    return true;
}

//Validates a string
function validateString(field, msg, min, max) {
    if (!min) { min = 1 }
    if (!max) { max = 65535 }

    if (!field.value || field.value.length < min || field.value.max > max) {
        alert(msg);
        field.focus();
        field.select();
        return false;
    }

    return true;
}

//Validates an email address
function validateEmail(email, msg, optional) {
    if (!email.value && optional) {
        return true;
    }

    var re_mail = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z])+$/;
    if (!re_mail.test(email.value)) {
        alert(msg);
        email.focus();
        email.select();
        return false;
    }

    return true;
}
</script>
<?php
} //close off else isset("some variable")
?>