Why does php sleep() pause execution of all scripts?

Question

I am trying to implement simple longpolling in php, my server code goes like this:

if (!isset($_SESSION["lastResult"])) {
    $_SESSION["lastResult"] = "";
}

while(true){
   $sql = "SELECT * from table";
   $result = mysqli_query($conn, $sql);

   $array = [];
   while ($row = $result->fetch_array()) {
        do something and insert each element into $array;
   }
   $jsonRes = json_encode($array);

   //if old array is different from the new one, that means information has been updated, 
   //so I need to echo new data to user.

   if (strcmp($jsonRes, $_SESSION["lastResult"]) == 0) {
        echo $jsonRes;
        $_SESSION["lastResult"] = $jsonRes;
        exit();
    }

    //If information hasn't changed, just sleep for 5 seconds and repeat next time.
    sleep(5);
}

and client code:

  const waitForData = () => {
    //this is just a js fetch function
    getDataHTTP().then(res => {
      this.setState({ itemsData: res });
      waitForData ();
    });
  };
  waitForData();

But what happens is in my network tab, I see my getData php script "pending" as it should, but its pending forever, and the other thing is that I can't really edit any other data on the webpage, because for some reason, which I assume is the sleep() function, All my php scripts stop working, and are forever "Pending".

Thanks in advance.

Edit:

Just for the record, $_SESSION variable works, all data is where it should be. Same is with SQL query, it returns everything correctly.

Edit2:

I am not getting any errors except max execution time.

Answer 1

Look at your code:

while(true){  // do this FOREVER unless we break the loop
   $sql = SELECT * from table;    // I assume we always get all rows?
   $result = mysqli_query($conn, $sql);
   $array = [];

   while ($row = $result->fetch_array()) {
        // Do stuff
        $array[] = $row;
   }
   // now array has all rows

   $jsonRes = json_encode($array);

   if (strcmp($jsonRes, $_SESSION["lastResult"]) == 0) {
        echo $jsonRes;
        $_SESSION["lastResult"] = $jsonRes;
        exit();
    }
    // so just sleep 5 seconds and do it all over again
    sleep(5);
}

Wouldn't it be easier to have a datetime column in the table, and if a select comes back with rows of an update time later than the last time the thing ran, then select * and return? whith no while true loop?