About PHP Functions

1. Built-in Functions

Functions that PHP provides by default.
Examples: strlen() (to get the length of a string), array_push() (to add an element to an array), etc.

Features

• Immediately usable as standard PHP features
• Hundreds to thousands of functions are available
• Fast and stable

2. User-Defined Functions

Functions created by developers themselves.
Used for code reuse and summarizing processing.

Example Syntax

function greet($name) {
  return "Hello, " . $name . "!";
}
echo greet("Taro"); // Hello, Taro!

Features

• You can encapsulate arbitrary processing
• Arguments and return values can be freely defined
• Improves readability and maintainability in large codebases

3. Anonymous Functions (Closures)

Functions without a name that are defined inline and can be assigned to variables or used as callback functions.

Example Syntax

$sayHello = function($name) {
  return "Hello, " . $name;
};
echo $sayHello("Alice"); // Hello, Alice

Features

• No name means it’s useful for one-time processing or callbacks
• Can capture variable scope as a closure
• Simple and flexible code usage

Summary of the Three Types of Functions

date()

Retrieves and formats date and time.

echo date("Y-m-d"); // e.g., 2025-06-25

strlen()

Gets the length of a string.

echo strlen("Hello"); // 5

strpos($a, $b)

Finds the position of the first occurrence of a character or substring within a string. Returns false if not found.

$pos = strpos("abcdef", "cd");
if ($pos !== false) {
    echo "Position is $pos";  // Displays 2 (counting from 0)
} else {
    echo "Not found";
}

array_push($arr, $a)

Adds an element to the end of an array.

$arr = [1, 2];
array_push($arr, 3);
print_r($arr); // [1, 2, 3]

array_merge($arr1, $arr2)

Combines two arrays into a new array.

$a = [1, 2];
$b = [3, 4];
$c = array_merge($a, $b);
print_r($c);  // [1, 2, 3, 4]

file_get_contents($filename)

Reads the contents of a file as a string.

$content = file_get_contents("sample.txt");
echo $content;

It is recommended to refer to the official documentation to learn how to use these functions.

1. return Signals the End of Function Processing

return is a command that says “return this value and end the function.”
The value written after return is returned to the caller.

Also, once return executes, the function ends there.
That means any code after return will not run.

function checkNumber($n) {
    if ($n < 0) {
        return "Negative number";  // Processing ends here, returns string
    }
    return "Zero or positive number";   // Returned if $n is 0 or more
    // Code after this line is not executed
}

echo checkNumber(-1);  // Displays "Negative number"
echo checkNumber(5);   // Displays "Zero or positive number"

2. Return Values Can Be Assigned to Variables and Used

Return values can be displayed directly or assigned to variables for further processing.

function add($a, $b) {
    return $a + $b;  // Returns the sum
}

$result = add(3, 7);  // Assign return value to $result
echo $result;         // Displays 10

3. Return Types Can Be Specified (PHP 7 and Later)

From PHP 7, you can declare the “type” of the return value in advance.
For example, a function that always returns an integer can be written like this:

function addInt(int $a, int $b): int {
    return $a + $b;  // Returns the result as an integer (int)
}

Specifying the type helps prevent mistakes where the function accidentally returns a value of a different type.

4. Use void for Functions That Don’t Return Values (PHP 7.1 and Later)

Functions that do not return a value can explicitly declare void to indicate “no return value.”

function sayHi(): void {
    echo "Hi!";  // Just displays a greeting, returns no value
}

void functions must not use return with a value.
(Using return; without a value is allowed, but returning a value is prohibited.)

1. Local Variables Can Only Be Used Inside Functions

Variables created inside a function are called “local variables.”
These variables can only be used inside that function and are destroyed after the function ends.

function myFunction() {
    $localVar = "This is a local variable";
    echo $localVar;  // Usable here
}

myFunction();
// echo $localVar;  // Error here (cannot use outside the function)

In other words, local variables cannot be viewed or modified from outside the function.

2. Global Variables Can Be Used Throughout the Program

Variables created outside functions are called “global variables” and are generally accessible from anywhere in the program.

However, you cannot directly use global variables inside functions.
You need a special method to access global variables inside functions.

3. Use the global Keyword to Use Global Variables Inside Functions

Inside a function, use the global keyword to tell PHP “this is a global variable” to use the global variable.

$globalVar = "Global variable";

function useGlobal() {
    global $globalVar;  // Makes $globalVar usable inside this function
    echo $globalVar;
}

useGlobal();  // Displays "Global variable"

If you don’t use global, the function’s inside is a separate “local world” where global variables are invisible.

4. Another Way: Use the $GLOBALS Array

PHP has a special array called $GLOBALS that contains all global variables.
Using this array inside a function allows access to global variables.

$var = "Test";

function test() {
    echo $GLOBALS['var'];  // Uses global variable $var here
}

test();  // Displays "Test"

You access $GLOBALS by specifying the variable name as a string key.