Impure Function

An impure function is a function that has at least one of the following characteristics:

Non-deterministic: for the same input, an impure function may return different output. For example, date() and random_string() return different values for each call, by definition.
Create side-effect: an impure function modify a state: a global variable, a row in a table, a lock, a file or data. This may affect any subsequent call to this function, leading to the previous characteristic.

A function that is not impure is a pure function.

This notion applies to functions, methods, native and custom functions, closures and arrow functions.

<?php

    echo random_string(0, 10);
    
    // an impure function: 
    // the static variable is a state that changes over time
    function foo() {
        static $x = 0;
        
        return $x++;
    }

?>

Documentation

See Also

Pure vs Impure Functions in Functional Programming – What’s the Difference?