[cellarius]

As a rule: You never, ever once address URL parameters directly like that. Opens you up for all kinds of vulnerabilities.

Read and implement: https://vborg.vbsupport.ru/showthread.php?t=119372

After that, you will have your parameter in a nice variable, for example: do.

And to your question - if a variable is not defined, you define it in your code. See PHP man isset. Along the lines of

if (!isset($do))
{
    $do = 1;
}

Or do

if (!isset($do) OR $do == 1))