How can the output of important variables in PHP be displayed during the debugging process to track program flow?

During the debugging process in PHP, it is essential to display the output of important variables to track the program flow and identify any issues. One way to do this is by using the `echo` or `var_dump` functions to output the values of variables at specific points in the code.

// Display the value of a variable using echo
$variable = "Hello, World!";
echo $variable;

// Display the structure and value of a variable using var_dump
$array = [1, 2, 3];
var_dump($array);