You could describe PHP as the primary interface to Apache. It would be straining the the metaphor a bit, but I'd say it's still a reasonable way to explain PHP'

You could describe PHP as the primary interface to Apache. It would be straining the the metaphor a bit, but I'd say it's still a reasonable way to explain PHP's popularity. mod_php enabled shared hosting with reasonable performance. If mod_perl had been usable in an untrusted shared environment, then PHP might never have taken off the way it did.