<input name="rollno" type="text" placeholder="Roll Number" required>
<input name="submit_view_details" type="submit" value="Proceed">

if(isset($_POST['submit_view_details']))
{
    $rollno = (int) $_POST['rollno'];
    $query = "select * from table1, table2 where table1.{$rollno}=table2.{$rollno}";
    $result=mysqli_query($connection,$query);
}

SELECT * FROM table1, table2 WHERE table1.rollno = table2.rollno
AND table1.rollno = {$rollno};

SELECT * FROM table1 NATURAL JOIN table2
WHERE rollno = {$rollno};

$query = "SELECT tbl1.*, tbl2.*
          FROM tbl1
          INNER JOIN tbl2 ON tbl1.id = tbl2.id
          WHERE tbl1.column = value ";

foreach ($pieces_2 AS $value) {
         $pieces_3[] ="(CONCAT_WS('|',$presql2) like '%$value%')";      //concat all columns from one table
 }
 $serch_jyouken = implode(" and ",$pieces_3);       // for multiple keywords
 $result1 = mysql_query("select p.p_no from pfr_data p where (" .$serch_jyouken .")");
 $res1 = array();
 while($r1 = mysql_fetch_array($result1){
         $res1[] = $r1['p_no'] ;        //fetch primary key from table and store it into array
 }
 foreach ($pieces_2 AS $value) {
         $pieces_4[] ="(CONCAT_WS('|',$presql3) like '%$value%')";      // same as above
 }
 $serch_jyouken1 = implode(" and ",$pieces_4);
 $result2 = mysql_query("select p2.p_no from pfr_mod_inform p2 where (" .$serch_jyouken1 .")" );
$res2 = array();
 while($r2 = mysql_fetch_array($result2)){
         $res2[] = $r2['p_no'];
 }
 $res_mrg = array_merge($res1 , $res2);     //merge array
 $result = implode("','",$res_mrg );        // array to sring
 $sql5 = $presql ." from pfr_data p where p.p_no in ('$result') order by p.section_p,p.status,p.no";