TestPercentage.php

<?php

$percentages = [
    'Milk' => 50,
    'Coffee' => 30,
    'Water' => 20,
];
$percentSum = 0;
foreach ($percentages as $valuePercent) {
    $percentSum = $percentSum + $valuePercent;
}

if ($percentSum > 100) {
    echo 'The sum of the percentages cannot be more than 100. Current sum is ' . $percentSum;
    exit;
}


$boundaries = [];
$last = null;
foreach ($percentages as $name => $chance) {
    //if there was no last boundary added
    if($last === null){
        $boundaries[$name . '_down'] = 1;
    } else {
        $boundaries[$name . '_down'] = $last + 1;
    }
    $boundaries[$name . '_up'] = $last + $chance;

    //update last reached number
    $last = $boundaries[$name . '_up'];
}

echo 'Boundaries:<br>';
foreach ($boundaries as $boundaryKey => $boundaryValue) {
    echo $boundaryKey . ' boundary is ' . $boundaryValue . "<br>";
}


$randomNumber = rand(1, 100);

echo '<br>';

foreach ($percentages as $name => $key) {
    if ($boundaries[$name . '_down'] <= $randomNumber && $randomNumber <= $boundaries[$name . '_up']) {
        echo "You got: " . $name . '<br>';
        echo 'Lower boundary is: ' . $boundaries[$name . '_down'] . '<br>';
        echo 'Your number was: ' . $randomNumber . '<br>';
        echo 'Upper boundary is: ' . $boundaries[$name . '_up'] . '<br>';
    }
}