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Código PHP:
<form name="myform" action="checkboxes.asp" method="post">
<b>Your Favorite Scripts Languages</b><br>
<input type="checkbox" name="list" value="1">Java<br>
<input type="checkbox" name="list" value="2">Javascript<br>
<input type="checkbox" name="list" value="3">Active Server Pages<br>
<input type="checkbox" name="list" value="4">HTML<br>
<input type="checkbox" name="list" value="5">SQL<br>
<input type="button" name="CheckAll" value="Check All"
onClick="checkAll(document.myform.list)">
<input type="checkbox" name="UnCheckAll" value="Uncheck All" onClick="uncheckAll(document.myform.list, this)">
<br>
</form>
function uncheckAll(field, element) {
var check;
if(element.checked)
check= true;
else
check= false;
for (i = 0; i < field.length; i++)
field[i].checked = check;
}