json - PHP try...catch failing to "catch"

I have the following code that to which the file is an
ajax call:

try {
 echo
            json_encode(exif_read_data($_POST["url"]));
} catch (Exception $e)
            {
 echo "Invalid
            URL";
}

Its
purpose is to accept an image url and return the JSON-encoded EXIF data, and when there
is a "correct" url, the
json_encode(exif_read_data($_POST["url"])); works just
fine.

However, when I intentionally
pass it a "bad" url in the jQuery ajax call
"url" : "url1", or in the php file directly
..exif_read_data("bob"), I receive this error in my console
log:

Warning: exif_read_data()
            [function.exif-read-data]: Unable to open file in ../public_html/_photos/ajax/exif.php
            on line 3
false