Advanced PHP Functions: Arguments, Recursion, Built-ins & Errors

1. Passing Arguments: By Value vs By Reference

When you call a function with arguments, PHP can pass the data in two fundamentally different ways:

a) Pass by Value (Default) Definition: A copy of the variable's value is passed to the function. Any changes made inside the function do NOT affect the original variable in the calling scope.

function addFive($num) {
  $num += 5;           // Only modifies the local copy
}
$val = 10;
addFive($val);
echo $val; // Output: 10 (original is UNCHANGED)

b) Pass by Reference Definition: The memory address (reference) of the variable is passed. Changes inside the function DO affect the original variable. Use the & symbol before the parameter.

function addFive(&$num) {  // & means "pass by reference"
  $num += 5;
}
$val = 10;
addFive($val);
echo $val; // Output: 15 (original IS changed)

Comparison Table: By Value vs By Reference

2. Recursion

Definition: Recursion is a technique where a function calls itself to solve a problem. Every recursive function must have a Base Case — a condition that stops the recursion — to prevent an infinite loop.

Structure of a Recursive Function:

1. Base Case: The condition that terminates the recursion.
2. Recursive Case: The function calls itself with a modified argument, moving closer to the base case.

Example 1: Factorial of a Number

function factorial($n) {
  if ($n <= 1) return 1;           // Base case: 1! = 1 and 0! = 1
  return $n * factorial($n - 1);   // Recursive call
}
echo factorial(5); // Output: 120 (5 * 4 * 3 * 2 * 1)

Trace: factorial(5) → 5 factorial(4) → 5 4 factorial(3) → ... → 54321 = 120*

Example 2: Fibonacci Series

function fibonacci($n) {
  if ($n <= 1) return $n;
  return fibonacci($n - 1) + fibonacci($n - 2);
}
echo fibonacci(7); // Output: 13

3. Important Built-in String Functions

PHP has 1000+ built-in functions. The most commonly used ones for strings:

4. The mail() Function

Used to send an email directly from a PHP script. Requires a correctly configured mail server (e.g., Sendmail/SMTP on the server).

Syntax: mail(to, subject, message, [headers], [parameters])

$to      = "student@example.com";
$subject = "Your Exam Result";
$message = "Dear Student, You have passed your exam. Congratulations!";
$headers = "From: admin@college.edu";

if (mail($to, $subject, $message, $headers)) {
  echo "Email sent successfully!";
} else {
  echo "Email sending failed.";
}

5. PHP Errors: Types, Causes & Effects

An error is an issue encountered during script execution. PHP has four main error types:

Error Handling with die() and trigger_error():

// die() stops execution and prints a message
$conn = mysqli_connect("localhost", "root", "") or die("Connection Failed!");

// Custom error with trigger_error
if ($age < 0) {
  trigger_error("Age cannot be negative!", E_USER_WARNING);
}

Custom Error Handler:

function myErrorHandler($errno, $errstr) {
  echo "Error [$errno]: $errstr";
}
set_error_handler("myErrorHandler");

Study Deep: Professional Error Management

For University exams and professional PHP development, understanding how to manage errors in a live environment is a key differentiator.

The PHP Error Cycle

1. Detection: PHP engine identifies a syntax, runtime, or logic issue.
2. Reporting: Based on error_reporting settings, the error is flagged.
3. Handling: Custom handlers or default engine actions take over.
4. Logging: Errors are recorded to server logs for debugging without exposing them to users.

Best Practices for Exams: