您应该在新的php版本中使用mysqli对象，并尝试以下代码
先建立连接

<?php

$con = mysqli_connect("localhost","my_user","my_password","my_db");

$result = mysqli_query($con,"SELECT * FROM doctors");
$rows = mysqli_num_rows($result);
echo "There are " . $rows . " rows in my table.";

  $content = '<div class="row">
        <div class="col-lg-3 col-xs-6">
          <!-- small box -->
          <div class="small-box bg-aqua">
            <div class="inner">
              *<h3><?php echo "$rows"; } ?></h3>*

              <p>Doctors</p>
            </div>
            <div class="icon">
              <i class="ion ion-bag"></i>
            </div>
            <a href="http://localhost/medibed/doctor" class="small-box-footer">View Doctors <i class="fa fa-arrow-circle-right"></i></a>
          </div>
        </div>
        <!-- ./col -->

      </div>';
  include('../master.php');
?>

如果您只需要count，那么为什么不在查询中使用聚合函数count（*）。这对你有更好的帮助。在h3标记的代码中，可以直接连接字符串，而不是再次使用php代码。这看起来更好，而且是结构化的。
试试这个：

<?php
$con = mysqli_connect("localhost","my_user","my_password","my_db");
$result = mysqli_query($con,"SELECT count(*) as total_rows FROM doctors");
$rows = $result->total_rows;
echo "There are " . $rows . " rows in my table.";
  $content = '<div class="row">
        <div class="col-lg-3 col-xs-6">
          <!-- small box -->
          <div class="small-box bg-aqua">
            <div class="inner">
          *<h3>'.$rows.'</h3>*
          <p>Doctors</p>
        </div>
        <div class="icon">
          <i class="ion ion-bag"></i>
        </div>
        <a href="http://localhost/medibed/doctor" class="small-box-footer">View Doctors <i class="fa fa-arrow-circle-right"></i></a>
      </div>
    </div>
    <!-- ./col -->
  </div>';
include('../master.php');
?>