How can the order of code execution affect variable visibility and functionality in PHP?

The order of code execution can affect variable visibility and functionality in PHP because variables need to be declared before they are used in order for them to be accessible within a certain scope. To ensure variables are visible and functional, make sure to declare them before they are used or accessed within the code.

// Declaring the variable before using it
$variable = "Hello World";

// Using the variable after declaring it
echo $variable;