Retrieving data from database and displaying in textbox when the button click

I search about the code for retrieving data from database when the retrieve button was click. But I can’t find any codes that similar in my situation.

Here is the flow of my system. I have plt_no and a retrieve button. The retrieve button is to check if the plt_no is already exist or not if it is already exist the data from the database will appear in the text area/textbox and if the plt_no did not exist there’s a message that the id number did not exist and I can input data in the textboxes.

I have an idea of using this code:
This code is to check if the Retrieve button is click and the plt_no is the data which the retrieve depend on. Like for example the user put plt_no 111 and when the user click the Retrieve button, if the 111 is already exist the data will appear in the textboxes but if not there’s a message appear saying the plt no did not exist.

$plt_no = $_POST['plt_no'];

And I have search code for retrieving data:

 // Connects to your Database 
 mysql_connect("", "username", "password") or die(mysql_error()); 
 mysql_select_db("Database_Name") or die(mysql_error()); 
 $data = mysql_query("SELECT * FROM friends WHERE pet='Cat'") 
 or die(mysql_error()); 
 Print "<table border cellpadding=3>"; 
 while($info = mysql_fetch_array( $data )) 
 Print "<tr>"; 
 Print "<th>Name:</th> <td>".$info['name'] . "</td> "; 
 Print "<th>Color:</th> <td>".$info['fav_color'] . "</td> "; 
 Print "<th>Food:</th> <td>".$info['fav_food'] . "</td> "; 
 Print "<th>Pet:</th> <td>".$info['pet'] . " </td></tr>"; 
 Print "</table>"; 

I attach my full code.

My problem is how can I used that code and if that code is the code I need to used for retrieving data.

I have an idea but i am luck of knowledge in coding

Any help is highly appreciated I ma willing to learn and share

Thank you