but the PHP part is mostly a cut and paste job.
If you add storing the E-Mail's contents into a database, and require graceful degradation of the form's validation functions (JS based and server-side checking), you have a full-blown interview test IMO. (Maybe it's even too much.)
Have them write it from scratch, obviously. It's an exercise in futility, but it is a test.
If the applicant does that without creating total spaghetti code, without opening any security holes (SQL injection / Mail injection / foolish use of includes....), and getting the character set issues right (UTF8/ISO....) they would already be very interesting candidates in my eyes. Bonus points if they can explain why they did what they did, and what potential security problems they worked around.
Bonus bonus points for the script sending out UTF-8 mails properly.
That's just my view, though: It could be too much for interviewing a Junior Programmer. I don't really know what that job description includes and doesn't include (plus, it probably varies from employer to employer.) Interested to see what others say.