Why does my box 12a code say PHM instead of one regular letter. It is not an option in the pull down menu so I don't know what to do.

....sounds like you need to  call that employer and ask what it represents, and if they are issuing corrected W-2 forms.

Box 12 are things the IRS requires and there is a strict format.  Box 14 are things that are mostly informational to the taxpayer, and there is no strict format for box 14.  Some things that might go in box 14 are taxable on a state level, but other items are just for information.  So there are two basic possibilities.

1. This is a required box 12 item with the wrong code.  You need to know the right code,.

2. This is a box 14 item put in the wrong place.  You can ignore it on your return or enter it in box 14 instead of box 12.  

You don't necessarily need a corrected W-2, but you do need to know what the code is for.