What are common reasons for PHP variables to appear empty or undefined, despite being populated in the code?

Common reasons for PHP variables to appear empty or undefined despite being populated in the code include scope issues, typos in variable names, or using the variable before it is assigned a value. To solve this issue, ensure that the variable is declared in the correct scope, double-check the variable names for any typos, and make sure the variable is assigned a value before trying to access it.

// Example code snippet to demonstrate fixing empty or undefined variables
$variable = "Hello, World!"; // Assign a value to the variable

// Check if the variable is empty or undefined before using it
if(isset($variable)){
    echo $variable; // Output: Hello, World!
} else {
    echo "Variable is empty or undefined.";
}