What is the usage of the isset function in PHP?

The isset function is used to check if a variable is set and not null. It takes one or more parameters and returns a boolean value indicating whether the variable has been set.

For example, the following code example demonstrates how to use the isset function:

$name = "John";

if (isset($name)) {
    echo "变量已设置。";
} else {
    echo "变量未设置。";
}

The output will be “Variable is already set.” because the variable $name has already been set.

The isset function can also be used to check if an element is set in an array. For example:

$fruits = array("apple", "banana", "orange");

if (isset($fruits[0])) {
    echo "数组元素已设置。";
} else {
    echo "数组元素未设置。";
}

The output will be “Array element already set.” because the first element of the array $fruits has already been set.

Note: The isset function will return true for empty strings, 0, and false, as they are considered set values.