<?php
//For 75% Display Chance
//Create random number between 1 and 4
$number = mt_rand(1, 4);
$remainder = $number % 4;
//The Math:
//If $number is 1 then remainder is 3 (25% of the time - Do Nothing)
//If $number is 2 remainder is 2 (runs code below - 25%)
//If $number is 3 remainder is 1 (runs code below - 25%)
//If $number is 4 remainder is 0 (runs code below - 25%)
//last 3 cases add up to 75% chance of code running
if ($remainder !== 3) { ?>
	PUT AD CODE HERE
<?php } ?>

<?php
//For 50% Display Chance
//Create random number between 1 and 2
$number = mt_rand(1, 2);
$remainder = $number % 2;
//The Math:
//If $number is 1 then remainder is 1 (50% of the time - Do Nothing)
//If $number is 2 then remainder is 0 (runs code below - 50%)
if ($remainder == 0) { ?>
	PUT AD CODE HERE
<?php } ?>

<?php
//For 25% Display Chance
//Create random number between 1 and 2
$number = mt_rand(1, 4);
$remainder = $number % 4;
//The Math:
//If $number is 4 then remainder is 0 (runs code below - 50%)
if ($remainder == 0) { ?>
	PUT AD CODE HERE
<?php } ?>