In this script (very easy script), we have 3 ads for rotation
first ad we want it to display 70%
second ad we want to display 20%
and the last one we want to display 10%

What to do

1. Random number 1-100
- if result = 1-70 (we have opportunity to display ad 1, 70%)
- if result = 71-90 (we have opportunity to display ad 2, 20%)
- if result = 91-100 (we have opportunity to display ad 3, 10%)

Example

In this script use rand(1, 100); to random number between 1 -100 and keep result in variable name "$result_random" if result = 85 ad 2 will display.

<?php

// random number 1 - 100
$result_random=rand(1, 100);

// if result less than or equal 70, display ad 1 (70%)
if($result_random<=70){
echo "Display ad1";
}

// if result less than or equal 90, display ad 2 (20%)
elseif($result_random<=90){
echo "Display ad2";
}

// if result less than or equal 100, display ad 3 (10%)
elseif($result_random<=100){
echo "Display ad3";
}

?>