PHP in a Nutshell

Checking Whether a Variable Is Set: isset()

Although most functions are covered in Chapter 7, you need to know the isset() function (literally, "is a variable set?") to make the most of this chapter. To use the function, send it a variable as the only parameter, and it will return true or false depending on whether the variable has a value assigned to it. For example:

    $foo = 1;
    if (isset($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.