Código PHP:
document.onkeydown = function(){
if((window.event && window.event.keyCode == 122) || (window.event && window.event.keyCode == 116) || (window.event && window.event.keyCode == 8) || (window.event && window.event.keyCode == 121) || (window.event && window.event.keyCode == 115) || (window.event && window.event.keyCode == 117)){
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 505){
return false;
}
}