[DJMaze]

You need a database table

CREATE TABLE hitcounter (
  ip VARCHAR(15) NOT NULL DEFAULT '',
  hits INT NOT NULL DEFAULT 1
);

PHP Code:

$db = mysql_connect('localhost', 'username', 'password');
if (!$db) {
    die(mysql_error());
}
if (!mysql_select_db('database_name', $db)) {
    die(mysql_error($db));
}

$hits = 0;
$ip = $_SERVER['REMOTE_ADDR'];

if (!mysql_query("UPDATE hitcounter SET hits=hits+1 WHERE ip='$ip'", $db) || mysql_affected_rows($db) < 1)) {
    // IP isn't listed yet so add it
    mysql_query("INSERT INTO hitcounter VALUES ('$ip', '1')", $db);
}
// IP is inserted so now get the ammount of ip's visited
$q_result = mysql_query("SELECT COUNT(*) FROM hitcounter", $db);
if ($q_result) {
    $row = mysql_fetch_row($q_result);
    $hits = $row[0];
} 


MORE ADVANCED AND SECURE (less HD space abuse)
then change 1 php line into:

PHP Code:

$ip = ip2long($_SERVER['REMOTE_ADDR']); 


And the database table

CREATE TABLE hitcounter (
  ip INT NOT NULL DEFAULT 0,
  hits INT NOT NULL DEFAULT 1,
  PRIMARY KEY (ip)
);

In the database the IP could use 7 to 15 bytes '0.0.0.0' to '255.255.255.255' but when we use ip2long() the IP will be converted to an 32bit integer which is only 4 bytes.
So on every table entry you save 3 to 11 bytes of HD space.
this doesn't look much untill there are 1 million entries.
As benefit there's also a "PRIMARY KEY" field which actualy protects the table from inserting duplicate IP's so each table entry has a unique ip.

As you see there's also a "hits" counter.
This actualy ads another feature. You can count the exact ammount of page hits instead of unique ip's.

PHP Code:

mysql_query("SELECT SUM(hits) FROM hitcounter", $db); 


This counts the total ammount of views you recieved.

PHP Code:

$hits = $visitors = 0;
$q_result = mysql_query("SELECT COUNT(*) FROM hitcounter", $db);
if ($q_result) {
    $row = mysql_fetch_row($q_result);
    $visitors = $row[0];
    mysql_free_result($q_result);
}
$q_result = mysql_query("SELECT SUM(hits) FROM hitcounter", $db);
if ($q_result) {
    $row = mysql_fetch_row($q_result);
    $hits = $row[0];
    mysql_free_result($q_result);
}
echo "We recieved $hits page views by $visitors visitors"; 


the output will be like

We recieved 15601215 page views by 49 visitors