(function($){
$.fn.checkgroup = function(options){
//merge settings
settings=$.extend({
groupSelector:null,
groupName:'group_name',
enabledOnly:false
},options || {});

var ctrl_box=this;


//allow a group selector override option
var grp_slctr = (settings.groupSelector==null) ? 'input[name='+settings.groupName+']' : settings.groupSelector;

//grab only enabled checkboxes if required
if(settings.enabledOnly)
{
grp_slctr += ':enabled';
}

//attach click event to the "check all" checkbox(s)
ctrl_box.click(function(e){
chk_val=(e.target.checked);
$(grp_slctr).attr('checked',chk_val);
//if there are other "select all" boxes, sync them
ctrl_box.attr('checked',chk_val);
});
//attach click event to checkboxes in the "group"
$(grp_slctr).click(function(){
if(!this.checked)
{
ctrl_box.attr('checked',false);
}
else
{
//if # of chkbxes is equal to # of chkbxes that are checked
if($(grp_slctr).size()==$(grp_slctr+':checked').size()){
ctrl_box.attr('checked','checked');
}
}
});
//make this function chainable within jquery
return this;
};
})(jQuery);

<input type='checkbox' class='checkall'>checkall<br>
<input class='groupclass' name='group' type='checkbox'>chk1<br>
<input class='groupclass' name='group' type='checkbox'>chk2<br>
<input class='groupclass' name='group' type='checkbox'>chk3<br>
<input class='groupclass' name='group' type='checkbox'>chk4<br>
<input type='checkbox' class='checkall' id="checkall">
<?php
$(function() {
$("#checkall").click(function() {
$('.checkall').checkgroup({groupSelector:'.groupclass',enabledOnly:true});
});
});

<?php
$(function() {
$("#checkall").click(function() {
$('#checkall').checkgroup({groupName:'group'});
});
});