how to generate custome error message

how to generate custome error message
<?php
$a = 'apple';
if ($a == 'apple'){
echo 'YES';
} else {
throw new Exception;	inbuilt class throw to generate errors
}
?>
$age = 18;
try {	if try failed, go to else, it has throw , it will call catch, at that it wil execute
throw	whenever else comes comes to thtrow
}	and execute in catch
catch() {
}
<?php
$age = 18;
try {
if ($age  >= 18){
echo ' you can vote';	if it is not age is equalto 18
} else {	throw the error as exception with text
throw new Exception ('YOU CANT ENTER');	whats we given text
}	that will catched by catch with
catch (Exception $ex){
echo $ex -> getmessage();	automatically get by the message function and loads to ex and prints
}
?>

how to check it is age below the age , u cant enter

condional enter and generate error