move php unified builder to google-gae-18 stack
move php unified builder to google-gae-18 stack

PiperOrigin-RevId: 452840277
Change-Id: I7a650c45f7121d1573a2d633f98de3842e2a84c9

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks