Which function would you use to check if a variable is set in PHP?

The function that is used to check if a variable is set in PHP is isset(). This function takes a variable as an argument and returns true if the variable is indeed set and is not null. It is commonly used to ascertain whether a variable has been initialized and contains a value, making it essential for handling potential errors in code where a variable might not be defined.

Using isset() helps in controlling the flow of the program by allowing developers to check if they can safely use a variable before doing operations on it, thereby preventing notices or warnings in cases where the variable is not set.

The other functions mentioned do not serve the same purpose: check() is not a standard PHP function for variable evaluation, defined() is specifically used to check if a constant has been defined, and exists() does not exist as a built-in PHP function for variable checking. Therefore, isset() is uniquely positioned for this functionality within the language.