如何使用此函数从mysql和PHP获取多行

I have this function which returns only one row, How can I modify the function so that it returns more than one row?

public function getVisitors($UserID)
{   
$returnValue = array();
$sql = "select * from udtVisitors WHERE UserID = '".$UserID. "'";

$result = $this->conn->query($sql);
if ($result != null && (mysqli_num_rows($result) >= 1)) {
$row = $result->fetch_array(MYSQLI_ASSOC);
if (!empty($row)) {
$returnValue = $row;
}
}
return $returnValue;
}

I would suggest storing them in an associative array:

$returnValue = array();

while($row = mysqli_fetch_array($result)){

    $returnValue[] = array('column1' => $row['column1'], 'column2' => $row['column2']); /* JUST REPLACE NECESSARY COLUMN NAME AND PREFERRED NAME FOR ITS ASSOCIATION WITH THE VALUE */

} /* END OF LOOP */

return $returnValue;

When you call the returned value, you can do something like:

echo $returnValue[0]['column1']; /* CALL THE column1 ON THE FIRST SET OF ARRAY */
echo $returnValue[3]['column2']; /* CALL THE column2 ON THE FOURTH SET OF ARRAY */

You can still call all the values using a loop.

$counter = count($returnValue);

for($x = 0; $x < $counter; $x++){
    echo '<br>'.$rowy[$x]['column1'].' - '.$rowy[$x]['column2'];
}

There is a function in mysqli to do so, called fetch_all(), so, to answer your question literally, it would be

public function getVisitors($UserID)
{   
    $sql = "select * from udtVisitors WHERE UserID = ".intval($UserID);
    return $this->conn->query($sql)->fetch_all();
}

However, this would not be right because you aren't using prepared statements. So the proper function would be like

public function getVisitors($UserID)
{   
    $sql = "select * from udtVisitors WHERE UserID = ?";
    $stmt = $mysqli->prepare($sql);
    $stmt->bind_param("s", $UserID);
    $stmt->execute();
    $res = $stmt->get_result();
    return $res->fetch_all();
}