I think the best way is to use the before_wp_tiny_mce() hook. Then, you can define the url in PHP; and pass it to the page so it is available to TinyMCE.

function sgp_before_wp_tiny_mce() {

    ?>
    <script type="text/javscript">
        var sgp_plugin_url = "<?php echo plugins_url('shortcode_generator_popup.php'); ?>";
    </script>
    <?php
}
add_action('before_wp_tiny_mce', 'sgp_before_wp_tiny_mce');

You may need to change the plugins_url() to match the appropriate location of your php file.

Then, you can use the variable in your plugin tinymce script to get the location.

ed.windowManager.open({
    file : sgp_plugin_url,

I haven't tested this... it's from memory. Let me know if you encounter any issues and I'll adjust accordingly.

I think the best way is to use the before_wp_tiny_mce() hook. Then, you can define the url in PHP; and pass it to the page so it is available to TinyMCE.

function sgp_before_wp_tiny_mce() {

    ?>
    <script type="text/javscript">
        var sgp_plugin_url = "<?php echo plugins_url('shortcode_generator_popup.php', __FILE__); ?>";
    </script>
    <?php
}
add_action('before_wp_tiny_mce', 'sgp_before_wp_tiny_mce');

You may need to change the plugins_url() to match the appropriate location of your php file.

Then, you can use the variable in your plugin tinymce script to get the location.

ed.windowManager.open({
    file : sgp_plugin_url,

I haven't tested this... it's from memory. Let me know if you encounter any issues and I'll adjust accordingly.