If you want php set a default variable value, but only if you yourself (in a particular case) set (before) another value of that variable, you can use a code like this:
$your_variable = $your_variable ?? $a_default_value;
That means: if you doesn’t define another value (with a code like $your_variable = "some_particular_value" ), the value of $your_variable will be $a_default_value.
Otherwise it will be "some_particular_value" .