The Middle East versions of Photoshop support Arabic fonts, but unfortunately the rest of editions do not cover this feature properly . But don't wory you are still able to use a trick to get the job done! Here is the way to type/write Arabic/Farsi/Persian in Photoshop CS3/CS4/CS5.
Simply download the following template and unzip it in a preferred location. Open the PSD file in Photoshop and start typing Arabic/Farsi! You may wish to customize the text or move it between different layers or images , which is absolutely possible.
Good luck!
Download:
Arabic-Farsi-Template-For-Photoshop.zip