$res = mysql_fetch_row(mysql_query("SELECT COUNT(*) FROM name"));

$array = range(1, $res[0]);

shuffle($array);

$row1 = mysql_fetch_array(mysql_query("SELECT login FROM name WHERE id = $array[0]"));

$row2 = mysql_fetch_array(mysql_query("SELECT login FROM name WHERE id = $array[1]"));

$row3 = mysql_fetch_array(mysql_query("SELECT login FROM name WHERE id = $array[2]"));

$row4 = mysql_fetch_array(mysql_query("SELECT login FROM name WHERE id = $array[3]"));

$img_url = "pic2.png";

$validate = "Правильно";

$arr = array(

'btn_1'    =>  $row1['login'],

'btn_2'    =>  $row2['login'],

'btn_3'    =>  $row3['login'],

'btn_4'    =>  $row4['login'],

'validate' =>  $validate,

'answer'   =>  5,

'src'      =>  $img_url

);

echo json_encode($arr);