What is the purpose of the isset function in PHP?

The isset function in PHP is used to check if a variable has been set and is not null. It determines whether a variable has been initialized or exists. If the variable has been initialized and is not null, isset will return true; otherwise, it will return false.

Using the isset function can prevent errors when accessing undefined variables. It is commonly used for form processing, checking if keys exist in an array, and detecting if variables have been assigned values.