This is my first php program!

30
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Chapter 2: Language Basics
Use the
is_null( )
function to test whether a value is
NULL
—for instance, to see
whether a variable has a value:
if (is_null($x)) {
// $x is NULL
}
Variables
Variables in PHP are identifiers prefixed with a dollar sign (
$
). For example:
$name
$Age
$_debugging
$MAXIMUM_IMPACT
A variable may hold a value of any type. There is no compile- or runtime type check-
ing on variables. You can replace a variable’s value with another of a different type:
$what = "Fred";
$what = 35;
$what = array('Fred', '35', 'Wilma');
There is no explicit syntax for declaring variables in PHP. The first time the value of
a variable is set, the variable is created. In other words, setting a variable functions as
a declaration. For example, this is a valid complete PHP program:
$day = 60 * 60 * 24;
echo "There are $day seconds in a day.\n";
There are 86400 seconds in a day.
A variable whose value has not been set behaves like the
NULL
value:
if ($uninitialized_variable === NULL) {
echo "Yes!";
}
Yes
Variable Variables
You can reference the value of a variable whose name is stored in another variable.
For example:
$foo = 'bar';
$$foo = 'baz';
After the second statement executes, the variable
$bar
has the value
"baz"
.
Variable References
In PHP, references are how you create variable aliases. To make
$black
an alias for
the variable
$white
, use:
$black =& $white;
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