Revised code to make sure URL starts with http

<?php
// http://www.gravityhelp.com/forums/topic/custom-input-mask-for-a-valid-url#post-157749
// revised to handle multiple forms and their associated URL fields
add_filter('gform_pre_submission_filter_338', 'force_http_prefix');
function force_http_prefix($form) {
    $formfields = array(
        // add all your form IDs here with their corresponding field ID
        // for example, this will look for field 3 in form 338
        '338' => '3',
        '12' => '3',
        '2' => '10',
        '8' => '7'
        // for all 11 forms ...
        );

    // see if the form ID is one of the keys in our array
    // if so, then run the code to check the appropriate field
    if (array_key_exists($form['id'], $formfields)) {

        // create a variable for the input we want to modify for this form
        $input_id = 'input_' . $formfields[$form['id']];
        $haystack = strtolower(trim($_POST[$input_id]));

        // check for http:// at the beginning of the string
        $needle = 'http://';

        // if the http:// is not at the beginning of the string, add it and modify the POST value
        if (substr($haystack, 0, 7) <> $needle) {
            $_POST[$input_id] = $needle . $haystack;
        }
    }
    // return the form either way
    return $form;
}