You have some errors here. The f flag says to replace the message with the output from the filter (though the b restricts this action to just the body). The braces are also superfluous here. So I'd go with
:0b
* ^Subject.*Telemetry rotate$
! `php -f /path/to/script/script.php`
if indeed the plan is to (1) pass the body to the PHP script, (2) capture the script's output (this is what the `backticks` do) and (3) forward the message to the address captured (that's wat the ! action does).
If your intention is merely to pass the body to your script, that would be
:0b
* ^Subject.*Telemetry rotate$
| php -f /path/to/script/script.php
maybe also with a c flag if you want to continue to process the message after this point.
You'll notice that I took out the hard-coded path /usr/bin; hardcoding the path makes the script less portable, and makes it impossible (or at least extremely cumbersome) to replace php with a wrapper for debugging purposes. I'd recommend to simply make sure you set up your PATH correctly in production.
!forwards the mail to the address that the PHP script outputs. That's what the!at the start does. Is this what you intend? Did you intend to use|instead? Check thepromailrcmanual... – Kusalananda Dec 18 '18 at 18:02fflag with an!action. This is a corner case I have never seen before. I'm pretty sure the recipe doesn't do at all what you want; but your question really should spell out more explicitly what you do want. The idea that backticks "escape" a command is certainly nonsense. – tripleee Dec 18 '18 at 19:56