strcmp with matching strings took 0.207852 seconds
strcmp with non-matching strings took 0.215276 seconds
=== with matching strings took 0.067122 seconds
=== with non-matching strings took 0.057305 seconds

=== is the clear winner. Function calls are always slower than operators. This was with PHP5, may be different for earlier versions. Not sure about the memory impact but I assume it's less since the strings do not have to be copied for the function call.