php：当有一位数字时找不到最大值

I used the following codes to find the maximum value of marks

 <?php
  $sql = mysql_query("SELECT MAX(marks) FROM participant ");
  $fetch = mysql_fetch_row($sql);
  $max_m = $data[0];


  $sql = mysql_query("SELECT st_name FROM participant WHERE marks='$max_m'");
  $fetch = mysql_fetch_row($sql);
  $st = $data[0];
  echo $st;
  ?>

but when I insert a mark that is 2-9 to test, the output become the student with that marks.

How to solve the problem? It works when i use 1 and the number between 10 and 20.(the full mark is 20)