PHP MYSQL Auto complete/Autopopulate web forms

Start with learning few topics:
1. Javascript
2. AJAX
3. JSON

Lots of people would suggest you to learn JQuery as well. I will not suggest that. I will not argue against it either.

What would be the process:
1. Write your php code to get data from mysql. Finally echo them in JSON format. If you want to do it in easy way then aggregate all data in an array and then use json_encode function. example:

$result=mysql_query("SELECT * FROM sometable", $conn);
$outputrow=array();
$i=0;
while($row=mysql_fetch_assoc($result))
{
 $outputrow[$i]=$row;
 $i++;
}
echo json_encode($outputrow);

2. Write the Ajax code to call the module from your client.
3. On return parse the returned data and then put the data on their proper element.