Here is a quick way to express a chance of something happening.

This is an example in JavaScript that gives you a 1 in 3 chance of being true.

[true, false, false].sort(function(){ return Math.random() >= 0.5 ? 1 : -1 })[0]

Basically, we populate an array w/ true and false values and select a random element.

Here is the same idea in PHP but we'll create a never ending loop so we can see it in action and print to the screen

<?php
$chance = 50; // Let's compute a 50% chance 
$true_vals = array_fill(0, $chance, true); 
$false_vals = array_fill(0, 100 - $chance, false); 
$vals = array_merge($true_vals, $false_vals); 
$tmp = [];

while(true){
    $f = [];
    foreach(range(1,count($vals)) as $i){
            shuffle($vals); // grab and use random true/false value
            $f[] = $vals[0]; 
    }
    $f = array_filter($f); // remove false values
    $c = count($f); // get count of true values
    $tmp[] = $c; 
    echo array_sum($tmp) / count($tmp) . "%\n";  
}

Here is a StackOverflow thread discussing the topic

https://stackoverflow.com/questions/11552158/percentage-chance-of-saying-something/47502706#47502706