PHP in filter xml

Good Morning,

I'm creating a filter for one of my forms. I'm currently using php to get the userId, and use that to filter a foreign key.
As you can see below, the code I am using is <?php echo 89; ?> (I've hard-coded it for testing purposes).

Sadly this is not working. Anyone know what I'm doing wrong? Is there a way to do it only via xml?

Thanks.

component_name/models/forms/myform.xml

<field name="filter_firstname" type="sql" class="inputbox"
onchange="this.form.submit();"
query="SELECT 0 AS `created_by`, '- Please select First name -' AS `first_name` UNION SELECT `created_by`, `first_name` FROM `#__applicationportal_names` WHERE `created_by`=<?php echo 89; ?>" key_field="created_by" value_field="first_name"
/>

Here is the solution if anyone else is interested.

/models/fields/foreignkey.php

Navigate to:

$query->select(
array(
$db->quoteName($this->key_field),
$db->quoteName($this->value_field)
)
)->from($this->table);

//Add the following code to append condition to the query.

if ($this->getAttribute('name') == 'firstname')
{
$query->where($db->quoteName('created_by') . ' = ' . (int) JFactory::getUser()->id);
}


Thanks to CC guys for the solution.