How to show error in PHP?

I am trying to run the exec function in order to fix any issue (if there is) with my IM codes.
However, it doesn't show any error.
As a matter of fact if there are any sorts of error, it won't let me know about it.
Can anyone please let me know how to enable it?
Laji.php :

Error:
Magick: unable to open image `null.png' :
No such file or directory @ error/bc/OpenBlob/2584
Magic: unable to open file `null.png' @ error/png.c/ReadPNGImage/3083.
Magick: missing an image filename `page.png' @ error/convert.c/ConvertImageCnd/2949.
