why php rand() function gives some time empty?

Hello When I try to use rand() function than it's give sometime empty like

// I want random value between 0-5000 , want 500 random value
 for ($i=0; $i < 500; $i++) { 
    $randNo = rand(0, 5000);

    if (in_array($randNo, $randArray)) {
        $randNo = InfiniteLoop($randArray ,$length_sec);
    }
    array_push($randArray,$randNo);
}
sort($randArray);

After that i print that array give some empty value.

//output
Array ( [0] =>  [1] => 523 [2] => 824 [3] ....so on )