Variables and Types in PHP

Data types

The basic types are integer (32 bit signed quantity), double (32 float) and string.
There is no boolean, instead 0, 0.0, and "" represent false.  Any other number or string represents true.  In particular, "0" is true even though 0 is false.  An example of the three types can be found at  variables.php .  Note how the double does not print as it is listed in the source.
Values that exceed the size of an int are automatically converted to a double.  So $a=100000000000000000000000001 is really a double.

Variable Names

Names must start with a letter or "_".  They may continue with a letter, number, or "_".  Variable names are case sensative, but built in function names are not.  WEIRD!

Constants

You can define constants within your code.
 
define("SCHOOL", "Northern Michigan");
echo(SCHOOL);
define("NL", "<br>\n");

Be convention constants have upper case names.  Accessing the value of a constant does NOT use a "$".
There are many predefined constants:  TRUE, FALSE, PHP_VERSION, PHP_OS, __FILE__, __LINE__.
Be careful with TRUE and FALSE, since not all values of TRUE and FALSE are the same.

Type Conversion

Variables have the type of their contents, and that can change on the fly.
Strings are converted to numbers in a numeric context. Numbers are converted to strings as needed, and in the obvious way.
Casting can be done just like in C++ or Java, or using the settype command.  The type of a variable can be determined by the gettype command.  The types for the gettype and settype commands are: integer, double, string, array, object, class, and "unknown type".

There are convience functions is_integer(), is_double(), etc.
$a = 11.2;         // a is a double
$b = (int)$a;      // b is an int
$c = (string)$a;   // c is a string

if (settype($a, "integer")) {
    echo("Converions YES");
}
else {
     echo("Conversion NO");
}
$d = gettype($a);

Useful Variable Functions

Name Example Explanation
isset() isset($a) Is $a defined.  Very useful to see if the form gave you and answer (as opposed to being blank).
unset() unset($a) Delete the variable, and free up space.
empty() exmpty($a) The opposite of isset().
intval() intval($a, 16) Returns $a when interprated as an int written in base 16.  Can do any base, including binary.