I want to validate integer form using PHP. Do anyone of you know how to validate it? my code like this ` Thông tin thành viên ">
<tr><td>First name : </td><td><input type="text" class="form-control" name="firstname"></td></tr>
<tr><td>Last name : </td><td><input type="text" class="form-control" name="lastname" /></td></tr>
<tr><td>User Name: </td><td><input type="text" class="form-control" name="username"/></td></tr>
<tr><td>Password: </td><td><input type="password" class="form-control" name="password"></td></tr>
<tr><td>Email: </td><td><input type="email" class="form-control" name="email"/></td></tr>
<tr><td>Phone: </td><td><input type="text" class="form-control" name="phone"/></td></tr>
<tr><td>Cấp độ : </td>
<td>
<select class="form-control" name="level">
<option selected>---Free choose---</option>
<option value="1">Administrator</option>
<option value="0">Member</option>
</select>
</td>
</tr>
<tr>
<td> <button type="submit" name="add" class="btn btn-primary btn-md">Thêm thành viên</button></td></tr>
</table>`
When people input the phone i want to check whether it is integer or not? can you help?