Here I've created a second empty array and used for loop with the first array which is having duplicates. It will run as many time as the count of the first array. Then compared with the position of the array with the first array and matched that it has this item already or not by using in_array. If not then it'll add that item to second array with array_push.
$a = array(1,2,3,1,3,4,5);
$count = count($a);
$b = [];
for($i=0; $i<$count; $i++){
if(!in_array($a[$i], $b)){
array_push($b, $a[$i]);
}
}
print_r ($b);