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