PHP - convertir a mysqli

$chk=mysql_num_rows(mysql_query("SELECT * FROM `requests` WHERE `sender`='$sender' AND `receiver`='$receiver'"));

* $conn= script de conexion a bd

$sql = "SELECT * FROM `requests` WHERE `sender`='$sender' AND `receiver`='$receiver'";

$rs = mysqli_query($conn, $sql);

$chk = mysqli_num_rows($rs);

$sql = "SELECT * FROM requests WHERE sender='$sender' AND receiver='$receiver'";

$rs = mysqli_query($conn, $sql);

$chk = mysqli_num_rows($rs);