Mexico City is fantastic for PHP internship positions, with 4 currently on Jobspotting. Why not set up a profile now to get individual recommendations? Most PHP jobs fall into the main categories Software Development and Programmer. Job seekers interested in these positions should familiarise themselves with fields like MySQL, Web Development and WordPress.
Of all companies hiring PHP expertise, none of these are startups. Why not have a look at jobs in another country to compare and contrast opportunities. Just use the Explore tab.