Calculate average of column from MYSQL query
You can use AVG like so:
SELECT AVG(P1_Score)
So in your case:
$gameswon = mysql_query("SELECT AVG(P1_Score) AS value_sum
FROM tblMatches
WHERE P1_ID LIKE '".$playerid."'");
Try using AVG() aggregate function instead of SUM
$gameswon = mysql_query("SELECT AVG(P1_Score) AS value_sum FROM tblMatches Where P1_ID LIKE '".$playerid."' . "GROUP BY XXXX");
and XXXX is the column that you want to get average for such as player