Not sure this is a good comparison because the PHP Foundation is "relatively" new and afaik there's no strong correlation to "the php development team" besides these people being paid and also work on it. PHP had a 20y history before this foundation, and the main difference is that it was never steered directly by one big company. Zend, yeah, but they had influence, they had neither started the project, nor ever had a long-stretching majority of people in the important positions. (Disclaimer: have not been an active part of the project for a couple years)

Flutter has much bigger scope than PHP Foundation. It's more fair to compare PHP Foundation scope to Dart Lang and Dart SDK. Flutter scope on top of that is Flutter Engine, Flutter Widgets, Flutter Framework, Flutter Packages, Dev Tools (IntelliJ + VSCode). PHP also support desktops/server environments instead of Flutter 6 platform (iOS, Android, Windows, Linux, MacOS, Web). Making crossplatform UI toolkit is the most labor intensive - PHP Foundation is not developing that. PHP had also 2 decades ahead of development.