3.2 Checking a variable is set: isset()This is NOT the latest copy of this book; click here for the latest version.
Although the majority of functions are covered in in the Functions chapter, you really need to get to grips with the isset() function (literally "is a variable set?") in order to make the most of this chapter. To make it work, you pass a variable in as the only parameter to isset(), and it will return true or false depending on whether the variable is set. For example:
<?php
$foo = 1;
if (isset($foo)) {
echo "Foo is set\n";
} else {
echo "Foo is not set\n";
}
if (isset($bar)) {
echo "Bar is set\n";
} else {
echo "Bar is not set\n";
}
?>
That will output "Foo is set" and "Bar is not set". Usually if you try to access a variable that isn't set, like $bar above, PHP will issue a warning that you are trying to use an unknown variable. This does not happen with isset(), which makes it a safe function to use.
As I said, flip ahead to the Functions chapter to read more about this function; for now, you just need to know this one.
Note that throughout this book (and throughout the programming world), "nonsense" variable names are used. Names like $foo, $bar, and $baz are the most common, but I often add $wom and $bat. Some argue against the use of these names, but the reality is that their use is so widespread - and that they can help liven up otherwise dull texts - and so I've used them liberally. That said, avoid them in your own code unless its for short-lived variables.
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