First off, you have no idea what this person is interested in learning or what they want to do with that knowledge. If all (s)he wants to do is learn web development, then getting into those other languages is not necessary at all.
That's right, the probability of using those other languages for web development is less than 1%. But I didn't say they should learn them to become a better
PHP programmer, but instead to become a better programmer in general.
C is a fairly low level language and C++ can be if you choose. A language like PHP hides a lot of the nitty gritty details; understanding some of those details
will make you a better PHP programmer.
For example, a PHP programmer can't figure out why their script is performing poorly. They've narrowed it down to a portion of the script that is doing repeated string concatenation. Someone who has used a more low level language will tell you the reason is probably because of repeated memory allocation / deallocation on a large chunk of memory.
Perhaps learning those worked for you, but not everyone is the same.
To be a better programmer, you really have to understand some concepts at the hardware level. You will not learn those concepts dealing with a "high-level" language like PHP.
Anyways, what I made was a recommendation to become a better programmer overall. If this person wants to work with PHP only, I'd second your recommendation on reading the manual in terms of objects.
Of course, you can't really appreciate the concept of objects until you create them in a language that doesn't support them, such as C.
