What best practice should be followed when setting default values for variables in PHP forms?

When setting default values for variables in PHP forms, it is best practice to check if the variable is set using the `isset()` function before assigning a default value. This ensures that the variable is only assigned a default value if it has not been previously set.

// Check if the variable is set, if not, assign a default value
$variable = isset($_POST['variable']) ? $_POST['variable'] : 'default_value';