PHP is a dynamically typed language, so although it expects a number, it will accept a string as it will try and parse that string into a number (e.g. you could pass it the string "2.23232" and it will work in the same way as if you pass it 2.23232 as a float). In this particular case, as the empty string can't be parsed into a number, it does indeed throw a PHP warning and treats it as a null input. My understanding is that it used to return 0 and that was considered a bug by some (although others, including the author of the function, decided that as the return type was a number it should ALWAYS return a number). The change that is under "dispute" in this case is that it has indeed been changed to return null, which is what most people now agree is the best way, and is consistent with other PHP functions.
I believe you are refering to strongly vs weakly typed, not statically vs dynamically typed. A strongly typed language would not try to parse the string into a number (although it would indeed accept a string, if it is dynamically typed as well).
