In interpreted languages such as PHP and Perl - the characters need to be converted to a number. I guess you could say that the less characters you need to convert the quicker it will be converted.

What you should however pay attention to is the types of variables you are using. When it comes down to values such as bytes there is an argument that storing them as a 32bit / 64bit integer is quicker than storing them as an 8bit integer as the whole thing can be thrown threw the processor at once. I am not sure of the turth in this but it hardly matters theis days .

As for floating point numbers - only use them if you need to. Single precision weighs in at 8 bytes and double prescision at 16 bytes. The whole number also comes in several urls. Floating point operations are somewhat slower than binary operations.