How to set the value of $_GET on page start as a value if its null? simple php

i want the the default value to be set to 5 min minutes meaning the value to be 1 when i start the page. not sure how to set the value for min before hand.

   <input type=radio name="min" value = "1"  <?php if($_GET["min"]==1){ echo "checked";}?>> 5 Min
   <input type=radio name ="min" value = "2"  <?php if($_GET["min"]==2){ echo "checked";} ?>> 20 Minutes
   <input type=radio name = "min" value ="3"  <?php if($_GET["min"]==3){ echo "checked";} ?> > 1 Hour