The behaviour of these functions is affected by settings in php.ini.
INI_ALL
Here's a short explanation of the configuration directives.
gd.jpeg_ignore_warning
Ignore warnings (but not errors) created by libjpeg(-turbo).
See also the exif configuration directives.
Image functions are very memory intensive. Be sure to set memory_limit high enough, if you are using the bundled version of the GD library.