php is built on "shared nothing" architecture. what does this imply?
if php is assigned a task which invokes system calls and some external programs, php will let the external programs manage their memory need. php won't take part in the process. it only asks the program to do the query and then send back the result. some programs which consume considerable amount of memory are database server, archiving programs, image processor, etc.
there is also another type of high resource usage which is likely to occure. it's called inappropriate system design. it could cause ineffective (or even false) algorithm, infinite loop, poor coding, etc. anyway, the last shouldn't contribute much to your program. even if the program you write is poor, it will still execute fast. this is related to the "shared nothing" architecture and optimization behind zvm.
in your case, it's not about your php code. simple include and echo won't take much resource except there is something wrong in the included file. if you're not advanced enough to analyze which code/program was it, you can view your site stat by visiting your cpanel. get the most popular page in your site, check how much bandwith is spent by opening/visiting the page and then you should be able to decide whether or not your host is cheating on you.
regards,