imagecreatefrompng error - how to detect and handle?

Question

In my script, I have the following lines:

$test = @imagecreatefrompng($name);
if ($test) { ... }

I am certain that $name represents an existing file on the disk, but I must handle cases where that file is not a valid PNG file (either because of a transfer error or because of a malicious user). I wish to handle such cases by not doing anything at all.

However, given the above code, my PHP interpreter stops on the first line with the following error message:

imagecreatefrompng() [function.imagecreatefrompng]: 'foobar.png' is not a valid PNG file

Shouldn't '@' have suppressed this error message and had the function return falseas described in the documentation? How can I tell PHP that I know an error might happen and not interrupt the execution?

Does http://fi2.php.net/manual/en/language.operators.errorcontrol.php#98895 help - are you using a custom error handler or something? If for some reason it absolutely won't work, you can always use output buffering to catch the error message, but that's a bit ugly. — Shadikka, Jan 18 '11 at 09:22
@Shaddika: you might want to post this as an answer, so that I may accept it. @Haim Evgi: it's not an exception. — Victor Nicollet, Jan 18 '11 at 09:26
I'm not sure that try/catch would work as an exception isn't being thrown. — Matt Lowden, Jan 18 '11 at 09:33

score 7 · Accepted Answer · answered Jan 18 '11 at 09:23

7

You could use mime_content_type on the file.

$image = 'file.png';
if(is_file($image) && mime_content_type($image_type) == 'image/png'){
    // Image is PNG
}else{
    // Not PNG
}

This will ensure the image is a file and a PNG.

answered Jan 18 '11 at 09:23

Matt Lowden

2,586
17
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Good stuff, does this do everything you need? – Matt Lowden Jan 18 '11 at 09:33
Well, the actual issue was correctly identified by Shaddika - there's a bug in my custom error handler that breaks the `@` construct. I've +1'ed your answer and will accept it unless Shaddika posts his comment as an actual answer instead in the next few minutes. – Victor Nicollet Jan 18 '11 at 09:36
How does this answer the question on how to detect errors with gd lib.. – Daniel W. Jan 21 '21 at 17:33

Ruslan Kabalin · Answer 2 · 2011-01-18T09:48:33.047

2

'@' is designed to suppresses errors, and you probably get the warning message.

You can do that using exceptions, e.g.

try {
    $test = imagecreatefrompng($name);
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
}

See more here

edited Jan 18 '11 at 09:48

answered Jan 18 '11 at 09:25

Ruslan Kabalin

6,580
2
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The link you provide has a note explaining why your answer does not apply here - "Internal PHP functions mainly use Error reporting, only modern Object oriented extensions use exceptions." – Victor Nicollet Jan 18 '11 at 09:32
Yep, but the same note explains how to make it work for "Error reporting" ones. – Ruslan Kabalin Jan 18 '11 at 09:42

imagecreatefrompng error - how to detect and handle?

2 Answers2