2.1 Introduction
In
everyday life, numbers are easy to identify. They're
3:00 P.M., as in the current time, or $1.29, as in the cost of a pint
of milk. Maybe they're like  , the ratio
of the circumference to the diameter of a circle. They can be pretty
large, like Avogadro's number, which is about 6 x
1023. In PHP, numbers can be all these
things.
However, PHP doesn't treat all these numbers as
"numbers." Instead, it breaks them
down into two groups:
integers and floating-point numbers.
Integers are whole numbers, such as -4, 0, 5, and 1,975.
Floating-point numbers are decimal numbers, such as -1.23, 0.0,
3.14159, and 9.9999999999.
Conveniently, most of the time PHP doesn't make you
worry about the differences between the two because it automatically
converts integers to floating-point numbers and floating-point
numbers to integers. This conveniently allows you to ignore the
underlying details. It also means 3/2 is
1.5, not 1, as it would be in
some programming languages. PHP also automatically converts from
strings to numbers and back. For
instance, 1+"1" is 2.
However, sometimes this blissful ignorance can cause trouble. First,
numbers can't be infinitely large or small;
there's a minimum size of 2.2e-308 and a maximum
size of about 1.8e308. If you need larger
(or smaller) numbers, you must use the BCMath or GMP libraries, which are
discussed in Recipe 2.14.
Next,
floating-point numbers
aren't guaranteed to be exactly correct but only
correct plus or a minus a small amount. Now, this amount is small
enough for most occasions, but you can end up with problems in
certain instances. For instance, humans automatically convert 6
followed by an endless string of 9s after the decimal point to 7, but
PHP thinks it's 6 with a bunch of 9s. Therefore, if
you ask PHP for the integer value of that number, it returns 6, not
7. For similar reasons, if the digit located in the 200th decimal
place is significant, floating-point numbers aren't
useful. Again, the BCMath and GMP libraries ride to the rescue. But,
for most occasions, PHP behaves very nicely when playing with numbers
and lets you treat them just as you do in real life.
|
