However I think it's fair to credit PHP with introducing generations of programers to the idea.
In the 1970s string interpolation really got going with languages like sh, cpp, and m4, which was popularized on many platforms by the "Software Tools" book (which in a sense is a slow buildup to the presentation of m4); m4 is Turing-complete entirely through string interpolation (more so even than Tcl decades later).
I suspect Perl had string interpolation from the beginning (1988), given its shell roots, but I don't have a copy of Perl 1 to test with.
For an overview of modern languages' string-interpolation syntax, I suggest https://www.rosettacode.org/wiki/String_interpolation_%28inc....
Hope this helps!
I'm reasonably sure that earlier versions of sh had some facility for this as well.
So, it's not really a PHP innovation, although some people might have seen it first there.
 https://www.in-ulm.de/~mascheck/bourne/v7/ -> "Parameter substitution"