Design a Form for landing page, integrate with simple API

I need a designed form for a landing page, that will use a POST command. The form will have some restrictions, for example: -Some fields are mandatory -Phone number must be 10 digits -If some fields in the form were not filled correctly, there should be some elegant message inside the form to alert the user about it (not an external browser message). The form should be submitted into a URL according to an easy API that I will provide. The design needs to be cross-browser compatible, from IE8 and up, but look nice. some PHP knowledge can be useful for the task. Budget is $15