You could consider alternative methods of authentication. Web based
authentication can allow applications to access thousands accounts, as one example the Demos and Tools site
can allow anyone to authorize it to generate reports upon their behalf. The use case for a service account is to grant permanent offline access to a Google Analytics Account, think of it as a trusted user that has programmatic access.
Alternatively you could allow for the growth organically, the end user could add the service account user through the web interface, however allowing growth beyond 100 accounts is not recommended as more expensive top level list operations may become unfeasible, such as segments.list.
What is the end use case for your PHP application? Is it a server application that generates reports for several hundred clients? Or is it an application which you could trigger and authorize at the request of a user who already has access to the accounts?