Which function is used to check if a variable is defined and not null?

Explanation

The `isset()` function determines if a variable is declared and is different than null. It returns false if the variable has been unset or is explicitly set to null. `empty()` checks for falsy values, while `defined()` is used for constants.

Key points

isset() returns false for null values
isset() returns true for any non-null value
defined() is strictly for constants

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