I'm glad to see a Belgian developer release his own pure PHP minification solution, but it can't compete yet with the compression rate of YUI compressor.
To test your solution, I took my code from https://github.com/jslegers/4inarow/blob/master/code.js and copy-pasted it into http://www.minifier.org/.
Before compression, the code is 3285 bytes. After compression, the code is recuded to 1908 bytes. When I compare the result with the result I get from http://refresh-sf.com/yui/, the code is reduced to 1233 bytes.
I would consider using your solution if the compression rate would be close enough to the compression rate of YUI compressor. It would also be nice to see the compression rate somewhere on the online interface and have a few configuration options, as it's done on http://refresh-sf.com/yui/ (see also #18).
To compare results, here's the output I got from my test :
Minifier
(function(doc){var start=function(){finished=false;changePlayer()},newGame=function(message){if(confirm(message)){start();forAllCells(emptyField)}},element=function(id){return doc.getElementById(id)},value=function(el){return element(el).innerHTML},cell=function(i,j){return element("c-"+i+"-"+j)},forAllCells=function(action){for(var t=1;t<7;t++){for(var counter2=1;counter2<8;counter2++){action(t,counter2)}}},sameColor=function(i,j){return testClass(i,j,players[current])},changePlayer=function(){element("c").innerHTML=players[current=(current+1)%2]},horizontalWon=function(i,j){for(var min=j-1;min>0;min--)if(!sameColor(i,min))break;for(var max=j+1;max<8;max++)if(!sameColor(i,max))break;return max-min>4},verticalWon=function(i,j){for(var max=i+1;max<7;max++)if(!sameColor(max,j))break;return max-i>3},diagonalLtrWon=function(i,j){for(var min=i-1,t=j-1;min>0;min--,t--)if(t<1||!sameColor(min,t))break;for(var max=i+1,t=j+1;max<7;max++,t++)if(t>7||!sameColor(max,t))break;return max-min>4},diagonalRtlWon=function(i,j){for(var min=i-1,t=j+1;min>0;min--,t++)if(t>7||!sameColor(min,t))break;for(var max=i+1,t=j-1;max<7;max++,t--)if(t<1||!sameColor(max,t))break;return max-min>4},colorField=function(i,j,color){cell(i,j).className=color},emptyField=function(i,j){colorField(i,j,'')},testClass=function(i,j,value){return cell(i,j).className==value},addCellBehavior=function(i,j){cell(i,j).onclick=function(j){return function(){if(!finished){for(var t=6;t>0;t--){if(testClass(t,j,'')){colorField(t,j,players[current]);if(horizontalWon(t,j)||verticalWon(t,j)||diagonalLtrWon(t,j)||diagonalRtlWon(t,j)){finished=true;newGame(wonMessage.replace("%s",players[current]))}else{changePlayer()}
break}}}}}(j)},players=[value("a"),value("b")],current=0,newGameMessage=value("n"),wonMessage=value("w"),finished;start();forAllCells(addCellBehavior);element("r").onclick=function(){newGame(newGameMessage)}})(document)
YUI Compressor
(function(t){var f=function(){i=false;g()},j=function(w){if(confirm(w)){f();l(r)}},d=function(w){return t.getElementById(w)},o=function(w){return d(w).innerHTML},b=function(x,w){return d("c-"+x+"-"+w)},l=function(y){for(var x=1;x<7;x++){for(var w=1;w<8;w++){y(x,w)}}},k=function(x,w){return h(x,w,e[n])},g=function(){d("c").innerHTML=e[n=(n+1)%2]},c=function(z,x){for(var y=x-1;y>0;y--){if(!k(z,y)){break}}for(var w=x+1;w<8;w++){if(!k(z,w)){break}}return w-y>4},u=function(y,x){for(var w=y+1;w<7;w++){if(!k(w,x)){break}}return w-y>3},s=function(A,x){for(var z=A-1,y=x-1;z>0;z--,y--){if(y<1||!k(z,y)){break}}for(var w=A+1,y=x+1;w<7;w++,y++){if(y>7||!k(w,y)){break}}return w-z>4},p=function(A,x){for(var z=A-1,y=x+1;z>0;z--,y++){if(y>7||!k(z,y)){break}}for(var w=A+1,y=x-1;w<7;w++,y--){if(y<1||!k(w,y)){break}}return w-z>4},v=function(y,x,w){b(y,x).className=w},r=function(x,w){v(x,w,"")},h=function(x,w,y){return b(x,w).className==y},q=function(x,w){b(x,w).onclick=function(y){return function(){if(!i){for(var z=6;z>0;z--){if(h(z,y,"")){v(z,y,e[n]);if(c(z,y)||u(z,y)||s(z,y)||p(z,y)){i=true;j(a.replace("%s",e[n]))}else{g()}break}}}}}(w)},e=[o("a"),o("b")],n=0,m=o("n"),a=o("w"),i;f();l(q);d("r").onclick=function(){j(m)}})(document);