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<!DOCTYPE html>
<html>
<body>
<?php
$a = 0;
// True because $a is empty
if (empty($a)) {
  echo "Variable 'a' is empty.<br>";
}
// True because $a is set
if (isset($a)) {
  echo "Variable 'a' is set.";
}
?>
</body>
</html>
Variable 'a' is empty.
Variable 'a' is set.