Do you have anything to back that up? An implementor of a PHP static compiler commented that the PHP runtime is horribly inefficient and is only able to appear fast in practice because the majority of the standard library is written in C. If you compare Ruby 1.9 to PHP on the Alioth tests at http://shootout.alioth.debian.org/u32/benchmark.php?test=all..., Ruby is very comparable in both memory and cPU usage and scores slightly higher overall, beating out PHP. MacRuby and Rubinius are eventually going to push Ruby performance even further, and right now JRuby already improves CPU performance over the standard runtime.
Rails as a framework may do some inefficient things, and I've heard that the next version of rails will address many of these inefficiencies, but as a language runtime Ruby isn't any sloppier performance-wise than PHP and might even already be better at this point.
1) "Ruby Scales, and It’s Fast" makes a point of talking about MRI not Yarv.
2) The fastest of those PHP programs relative to Ruby 1.9 (n-body and mandelbrot) don't seem to make any use of the PHP standard library - which undermines your "library is written in C" point.
3) If you look at the quad-core measurements you'll see PHP but you won't see Ruby 1.9 or MRI because (unlike PHP) no one has contributed Ruby programs that can make use of more than one processor.
Rails as a framework may do some inefficient things, and I've heard that the next version of rails will address many of these inefficiencies, but as a language runtime Ruby isn't any sloppier performance-wise than PHP and might even already be better at this point.