Functions

## Learning Objectives - Create user-defined functions - Understand parameters and return values - Work with variable-length argument lists - Master anonymous functions and closures - Use arrow functions ## Defining Functions ### Basic Function ```php ``` ### Function with Parameters ```php ``` ### Return Values ```php ``` ### Multiple Return Values (via Array) ```php ``` ## Parameters ### Default Parameter Values ```php ``` ### Positional vs Named Arguments (PHP 8) ```php ``` ### Type Declarations ```php ``` ### Nullable Types ```php ``` ### Void Functions ```php ``` ### Mixed Type (PHP 8) ```php ``` ## Variable-Length Arguments ### func_get_args() ```php ``` ### Spread Operator (...) ```php ``` ### Fixed + Variable Args ```php ``` ## Return Types ### Declaration ```php ``` ### Union Types (PHP 8) ```php ``` ## Anonymous Functions ### Basic Anonymous ```php ``` ### Using use() ```php ``` ### Changing Outer Variable ```php ``` ## Arrow Functions (PHP 7.4+) ### Basic Arrow Function ```php $x * 2; echo $double(5); // 10 ?> ``` ### Arrow with Multiple Args ```php $a + $b; echo $add(3, 4); // 7 ?> ``` ### Arrow with Multiple Expressions ```php $x * $factor + ($factor > 1 ? 5 : 0); echo $calculator(3); // 11 ?> ``` ### Arrow vs Anonymous ```php $x * $factor; // Anonymous - must use 'use' explicitly $anon = function($x) use ($factor) { return $x * $factor; }; ?> ``` ## Callable Type ### Using Functions as Arguments ```php ``` ### Type Declarations for Callables ```php print("Hi!\n"), 3); ?> ``` ## Callback Functions ### array_map Callback ```php ``` ### array_filter Callback ```php ``` ### usort with Callback ```php "Alice", "age" => 25], ["name" => "Bob", "age" => 30], ["name" => "Charlie", "age" => 20] ]; usort($people, function($a, $b) { return $a["age"] <=> $b["age"]; }); print_r($people); // Sorted by age ?> ``` ## Recursion ### Factorial ```php ``` ### Fibonacci ```php ``` ## Variable Functions ### Dynamic Function Calls ```php ``` ### Method Variants ```php $method(); // Calls $obj->myMethod() ?> ``` ## Built-in Functions Reference ### Math Functions ```php ``` ### String Functions ```php ``` ## Summary - Define functions with `function name() { }` - Parameters with defaults: `function($x = 10)` - Return values: `return $value;` - Type declarations for parameters and returns - Variable args: `...$args` (spread operator) - Anonymous: `$func = function() { }` - Arrow functions: `fn($x) => $x * 2` - Use `use($var)` to capture outer variables in closures - Use `&` to modify outer variables in closures - Recursion: function calls itself - Functions are first-class: can be assigned to variables