Which function is used to check if a variable has been declared and is not null?

Explanation

The isset() function determines if a variable is declared and is different than null. The empty() function checks if a variable is considered empty (e.g., 0, false, null, empty string). is_null() checks if a variable is explicitly null, and defined() is used for constants.

Key points

isset() returns false for null
isset() returns true for 0 or false
defined() is for constants only

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