Undefined Variable

Why This Happens

PHP raises this warning when you reference a variable that was never defined or is out of scope. This commonly happens when you misspell a variable name, forget to initialize it before a conditional block, or expect a variable from a different scope like inside a function.

The Problem

function greet() {
    echo "Hello, $username";
}
$username = "Alice";
greet();

The Fix

function greet(string $username) {
    echo "Hello, $username";
}
$username = "Alice";
greet($username);

Step-by-Step Fix

1. 1

   Identify the variable

   Look at the warning message to find which variable is undefined and on which line it is used.

2. 2

   Trace the assignment

   Check if the variable was assigned before the line that uses it. Look for typos, scope issues, or conditional branches that skip assignment.

3. 3

   Initialize or pass the variable

   Either initialize the variable with a default value, pass it as a function parameter, or use the global keyword if needed.