PHP filter code ( beginners' question )

[Truman]

I'm learning PHP from w3school tutorial and I need explenation about "!", for example take a look at this code:

 

<?php
$int = 100;

if (!filter_var($int, FILTER_VALIDATE_INT) === false) {
echo("Integer is valid");
} else {
echo("Integer is not valid");
}
?>

 

Why is there exclamation mark in this code?

And one more thing: if filter_validate_int ( or any other filter ) is false that is because the filter didn't catch any error and therefore, in this case integer, is valid. Did I get this correct?

[Ingolme]

If you've gone through the tutorial in order you must have seen the Operators section.