这很容易做到。注意下面代码中的注解。

$priorities = array(
    6=> 10,
    5=> 40,
    4=> 35,
    3=> 5,
    2=> 5
);

# you put each of the values N times, based on N being the probability
# each occurrence of the number in the array is a chance it will get picked up
# same is with lotteries
$numbers = array();
foreach($priorities as $k=>$v){
    for($i=0; $i<$v; $i++)  
        $numbers[] = $k;
}

# then you just pick a random value from the array
# the more occurrences, the more chances, and the occurrences are based on "priority"
$entry = $numbers[array_rand($numbers)];
echo "x: ".$entry;

创建一个介于1和100之间的数字。

If      it's <= 10       -> 6
Else if it's <= 10+40    -> 5
Else if it's <= 10+40+35 -> 4

等等...
注意：你的概率加起来不是100%。

最好的方法是生成一个0到100之间的数字，然后看看这个数字在什么范围内：

$num = rand(0, 100);
if ($num < 10) {
    $result = 6;    
} elseif ($num < 50) { // 10 + 40
    $result = 5;
} elseif ($num < 85) { // 10 + 40 + 35
    $result = 4;
} elseif ($num < 90) { // 10 + 40 + 35 + 5
    $result = 3;
} else {
    $result = 2;
}

注意，如果你的总概率不等于1，那么有时$result将是未定义的。