PHP - Php Average Number

I have a "Users" table. I would like to find out the average users sign up in total. This table has a "joined date" column so I can track how many users sign up in a single day.

For e.g.

August 16 - 10 users

August 17 - 20 users

Auguest 18 - 30 users

The total average of user sign ups would be 20 users based on the above results.

So I am wondering how can I create this function?

This is my starting query.

$get_users = $db->prepare("SELECT user_id FROM users");
$get_users->execute();
$result_users = $get_users->fetchAll(PDO::FETCH_ASSOC);
if(count($result_users) > 0) {
  foreach($result_users as $row) {
    $user_id = $row['user_id'];
  }
}

$sql2 = "Select element,
	SUM(CASE WHEN Month = 'Jan' THEN total ELSE 0 END ) AS Jan,
	SUM(CASE WHEN Month = 'Feb' THEN total ELSE 0 END ) AS Feb,
	SUM(CASE WHEN Month = 'Mac' THEN total ELSE 0 END ) AS Mac,
	SUM(CASE WHEN Month = 'Apr' THEN total ELSE 0 END ) AS Apr,
	SUM(CASE WHEN Month = 'May' THEN total ELSE 0 END ) AS May,
	SUM(CASE WHEN Month = 'Jun' THEN total ELSE 0 END ) AS Jun,
	SUM(CASE WHEN Month = 'Jul' THEN total ELSE 0 END ) AS Jul,
	SUM(CASE WHEN Month = 'Aug' THEN total ELSE 0 END ) AS Aug,
	SUM(CASE WHEN Month = 'Sep' THEN total ELSE 0 END ) AS Sep,
	SUM(CASE WHEN Month = 'Oct' THEN total ELSE 0 END ) AS Oct,
	SUM(CASE WHEN Month = 'Nov' THEN total ELSE 0 END ) AS Nov,
	SUM(CASE WHEN Month = 'Dec' THEN total ELSE 0 END ) AS Dis
	FROM tbl_ma GROUP BY element";
	$rs2 = mysql_query($sql2);
	$getRec2 = mysql_fetch_assoc($rs2);

<?php 
while ($row = mysql_fetch_assoc($rs2)) {
?>
<tr>
<td style="color:black;background-color:#f5efef"><div align="left"><?php echo $row['element']; ?></div></td>
<?php if( $year != '2014' ){ ?>
<?php if( 1 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Jan']; ?></div></td><?php endif; ?>
<?php if( 2 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Feb']; ?></div></td><?php endif; ?>
<?php if( 3 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Mac']; ?></div></td><?php endif; ?>
<?php if( 4 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Apr']; ?></div></td><?php endif; ?>
<?php if( 5 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['May']; ?></div></td><?php endif; ?>
<?php if( 6 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Jun']; ?></div></td><?php endif; ?>
<?php } ?>
<?php if( 7 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Jul']; ?></div></td><?php endif; ?>
<?php if( 8 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Aug']; ?></div></td><?php endif; ?>
<?php if( 9 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Sep']; ?></div></td><?php endif; ?>
<?php if( 10 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Oct']; ?></div></td><?php endif; ?>
<?php if( 11 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Nov']; ?></div></td><?php endif; ?>
<?php if( 12 <= $monthID ): ?><td><div align="center" style="color:black"><?php echo $row['Dis']; ?></div></td><?php endif; ?>
<td><div align="center" style="color:black"><?php //echo average; ?></div></td>
</tr>                                  
<?php } ?>

public function Update(Entry $e)
    {
        try
        {
            $query = "update entry set string = $e->string,delimiter=$e->delimiter where entryid= $e->id";
            $stmt = $this->db->mysqli->prepare($query);
            $stmt->bind_param('ssi',$e->string,$e->delimiter,$e->id);

            $stmt->close();
           

        }
        catch(Exception $ex)
        {
            print 'Error: ' .$ex->getMessage();
        }
    }