Nickname server for FiveM server v 1.1

<?
$limit = 5;
date_default_timezone_set('UTC');
$PHPfunc = $_GET['PHPfunc'];
if($PHPfunc=="get"){get();}else if($PHPfunc=="PC"){PC_Connect();}else if($PHPfunc=="CLient"){Client_Conncet();}

function get()
{
include('connectSQL.php');
$query = mysqli_query($sql, "SELECT * From players WHERE PC='" . $_GET['PC'] . "'");
if(mysql_num_rows($query)>0)
{
    $row = mysqli_fetch_array($query);
    echo $row['nick'];
}
else
{
    echo 'NULL';
}
}

function PC_Connect()
{
include('connectSQL.php');
$query = mysqli_query($sql, "SELECT * From players WHERE PC='" . $_GET['PC'] . "'");
if(mysql_num_rows($query)==0)
{
    mysqli_query($sql,"UPDATE tmp SET PC = '".$_GET['PC']."' , time = '". time()."' WHERE id = '0'");
}
}

function Client_Conncet()
{
include('connectSQL.php');
$query = mysqli_query($sql, "SELECT * From players WHERE nick='" . $_GET['nick'] . "'");
if(mysql_num_rows($query)==0)
{
    $query = mysqli_query($sql, "SELECT * From tmp WHERE id='0'");
    $row = mysqli_fetch_array($query);
    $time = time();
    $difference = $time - $row["time"];
    if($difference<$limit)
    {
        mysqli_query($sql,"INSERT INTO players (PC, nick) VALUES ('" . $row['PC']. "','" .$_GET['nick']. "')");
    }
}
}
?>