FormSubmit: not open source.
Simple Form: needs Ruby on Rails on the server.
I’m looking for something in PHP so that’s easy to install.
I think we are talking about two different things. OhMyForm is primarily a formbuilder (that is not php) and not so simple to implement and it has to be installed on your own server.
The ones I mentioned are web services developed for people running static pages, for the servers those pages use do not have dynamic components like php. (and that’s why those static pages are so fast and secure)
Airform: “What if you could use a service that gives you full control over the form, yet doesn’t require you to use anything on your server? Airform gives you the ability to create functional HTML forms without thinking about servers.”
Serverless Forms: “ServerlessForms is hosted in cloud and managed by us so that you don’t need to worry about form data management. Just focus on your website design and content. We will take care of your form submits”