Here is one:
<?php function p($c){$r=preg_replace(array("~^ +([^\n]+)~m",'~^-\s+(.*)$~m', "~-{4,}\r?\n~",'~(http(?:s)?)://([^\s]+)~i','~\n~'),array('<code>$1</code>','<li>$1', '<hr>','<a href=$1://$2>$2</a>','<br>'),$c);preg_match_all('~([AZ]\w+){2,}~',$r, $x);foreach(array_unique($x[0])as$m){$r=str_replace($m,x($m)? "<a href=?$m>$m</a>": "$m<a href=?e=$m>?</a>",$r);}return$r;}function f($f){@mkdir('wik');return @file_get_contents("wik/$fw");}function b($b){echo"<h1><a href=?$b>Backlinks $b" ."</a></h1><div id=c>";foreach(glob('wik/*.w')as$f){$f=substr($f,4,-2);if(strpos( f($f),$b)!==false)echo"<a href=?$f>$f</a><br>";}echo"</div>";}function x($f){ return file_exists("wik/$fw");}function e($p){$p=$p?$p:$_GET['e'];echo"<h1>Edit" ." $p</h1><form action='?$p' method=post><textarea name=c cols=50 rows=10>".f($p) ."</textarea><br><input type=submit value=Save>";}$p=preg_replace('~(e|b)=(.*)~' ,'',$_SERVER[QUERY_STRING]);$c=$_POST[c];$e=$_GET[e];$b=$_GET[b];if(!$p&&!$e)$p= 'MainPage';if($c){@file_put_contents("wik/$pw",htmlspecialchars($c));header( "Location: ?$p");}echo"<title>Wik Wiki</title>";if(!$e){if(!$b){echo x($p)? "<h1><a href=?b=$p>$p</a></h1><div id=c>".p(f($p))."</div><hr><a href=?e=$p>" ."Edit</a> | <small>Modified: ".date('dmY @ H:i:s', @filemtime("wik/$pw")) :e($p);}else b($b);}else e($e);
From http://c2.com/cgi/wiki?WikWiki . It does not have file uploads, but you can add it without bloating. I used it as a private notepad and it works well, you just need to make sure the web server has write permissions in the wik directory