Difference between float and double in php?

For PHP, they are the same. http://www.php.net/manual/en/language.types.float.php :

Floating point numbers (also known as "floats", "doubles", or "real numbers") can be specified using any of the following syntaxes: [...]

The confusing part is why gettype (which you shouldn't use, anyway) returns "double" instead of "float". The answer is http://de2.php.net/manual/en/function.gettype.php:

" double " (for historical reasons "double" is returned in case of a float , and not simply "float")

There is no difference in PHP. float, double or real are the same datatype.

At the C level, everything is stored as a double.
The real size is still platform-dependent.

See the manual for more details:
http://www.php.net/manual/en/language.types.float.php