isset

In PHP, isset is a language construct that checks whether a variable is defined and its value is not NULL. It returns a boolean value and can be used with multiple expressions in a single call.

When used with multiple operands, isset returns true only if all expressions are defined and not NULL. If any expression is undefined or NULL, the result is false. It short-circuits, evaluating operands left to right and stopping as soon as one evaluates to false. isset does not emit notices for undefined variables, which makes it useful for guarding against undefined indices or properties in user input or array data.

Common usage examples include simple checks like if (isset($name)) { ... } and more complex forms such as if (isset($data['key'])) { ... }. It also supports nested checks like isset($data['parent']['child']), which is false if any part of the chain is missing or NULL.

Important distinctions include the fact that isset tests for NULL; values such as 0, '', false, or an empty array are considered set. empty() is a different function that returns true for values that are considered empty, which can include 0 or an empty string. is_null() checks whether a value is NULL but may generate notices if the variable is not defined. array_key_exists('key', $array) checks for the existence of a key regardless of whether its value is NULL, providing a different behavior from isset in that case.

Notes: isset is widely used to validate input and guard array access, especially when handling user-provided data, to avoid undefined index warnings while allowing legitimate non-NULL values.