Determining Variable Undefinedness in PHP

In JavaScript, "document.createTouch !== undefined" checks for the undefinedness of "document.createTouch." Seeking an equivalent in PHP, let's explore ways to determine if a variable is undefined.

Using "isset()"

Unlike JavaScript, PHP does not have an explicit "undefined" keyword. Instead, you can use "isset()" to check if a variable has been defined. It returns true if the variable exists and false otherwise. For example:

$isTouch = isset($variable);

Handling Special Cases

It's important to note that "isset()" returns true even if the variable contains the value NULL. To check if a variable is undefined and not just empty or set to NULL, you can use the following:

if (!isset($variable) || is_null($variable)) {
  // $variable is undefined
}

Using "empty()"

Alternatively, you can use "empty()" to check if a variable is undefined or contains certain values, including the empty string, zero, NULL, and an empty array. However, "empty()" will not distinguish between undefined variables and those set to false.

$isTouch = empty($variable);