在PHP数组中的元素中查找随机元素？

        $stmt = $web_dbi->prepare($query);
        $stmt->execute();
        $result = $stmt->get_result();  
        $num_of_rows = $result->num_rows;   

        while($row=mysqli_fetch_array($result)){
          $results[] = $row;
        }

        $randomNumber = mt_rand(0,($num_of_rows-1));
        if($results[$randomNumber]['mysql_field']==anothervalue){
        ...

how do I access, at random, a given element of $results[]? Is this syntax correct in this scenario?

Your code looks fine. I personally prefer using array_rand to select a random element from the array:

$random_value = $results[array_rand($results)];

shuffle($results);
$item_to_use = array_pop ($results);

Not the most efficient but uses fewer lines of code.

Alternativley, you can use sql query to get random result.

SELECT field1, field2, field3 FROM table_name ORDER BY RAND()

Or to get random array element from PHP array try this,

echo $results[mt_rand(0, $num_of_rows - 1)]['mysql_field'];