ZEND PHP 5 Certification STUDY GUIDE

14 ” PHP Basics
}
echo ’myFunc!’;
$f = ’myFunc’;
$f(); // will call myFunc();
Clearly, this technique should be used with as much care as variable variables, as the
opportunities for mistakes and security issues it raises are quite significant.
Determining If a Variable Exists
One of the downsides of the way PHP handles variables is that there is no way to
ensure that any one of them will exist at any given point in the execution of a script.
This can introduce a range of problems—from annoying warnings if you try output
the value of a non-existent variable to significant security and functionality issues
when variables are unexpectedly unavailable when you need them.
To mitigate this problem, you can use the special construct isset():
echo isset ($x);
A call to isset() will return true if a variable exists and has a value other than NULL.
Constants
Conversely to variables, constants are meant for defining immutable values. Constants
can be accessed for any scope within a script; however, they can only contain
scalar values. Constant names, like variables, are case-sensitive; they also follow the
same naming requirements, with the exception of the leading $. It is considered best
practice to define constants using only upper-case names.
Here’s an example of constants at work:
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define(’EMAIL’, ’davey@php.net’); // Valid name
echo EMAIL; // Displays ’davey@php.net’
define(’USE_XML’, true);
if (USE_XML) { } // Evaluates to true