In the RFC, Ramsey argues that is not the case. PHP has not required a contributor-license agreement (CLA) to assign copyright to the project, and there is no implicit transfer of copyright to the project. However, there is an implied assignment of license:

  When someone contributes to an open source project, they own the copyright on their contributions, but unless they specify a different license covering their contributions (which is wholly valid, with examples including Derick Rethans's timelib, which is bundled within the PHP source code), it is implied they are granting use of their contributions under the same license terms as the project.
  
  [...] Typically, when changing the license on an open source project, one must gain approval from all copyright owners, since the rights granted might change under the terms of the new license. However, as described in this section and in other places in this document, changing to the Modified BSD License does not change any of the rights granted by contributors who are not the PHP Group or Perforce Software.