From what I can see, phpldapadmin isn't available in Debian Stretch because it relies on PHP5 and Stretch only has PHP7. I'm not sure why it is in Raspbian.
It may be possible to pull in the version from Raspbian Buster, which uses PHP7, but that means a bit of jiggery-pokery in the sources.list.d files and apt-preferences. If you follow this route, always take a backup of your system first. Preferable to another SD card, then boot up from the backup - don't touch the original. That way you can be sure that your backup worked and you have a working original to fall back on if something breaks.
FWIW, I'm running a couple of systems that have some packages from Buster installed (and even one with a few bits from Debian Sid, but I would strongly not recommend that).