Hello!
I have tested your pdf_open_image_file -function. 
It fails when I use gif-pictures that have a color-depth smaller than 16 bits per pixel. That means gifs with 16(4bpp) or 2 colors (b/w) will fail. With 256(16bpp) and 16,7M(24bpp) colors everything runs fine.
I used the following simple script for testing it:

<?PHP
           $fp = fopen("test.pdf", "w");
           $pdf = pdf_open($fp);
           $picname="test.gif";

           pdf_begin_page($pdf, 595, 842);

           $pim = pdf_open_image_file($pdf, "gif", $picname);
           pdf_place_image($pdf, $pim, 50, 100, 1);
           pdf_close_image($pdf, $pim);

           pdf_end_page($pdf);
           pdf_close($pdf);
           fclose($fp);
?>


The output of the script in case of errors was:

Warning: Could not open image in /usr/local/httpd/htdocs/regent/pdftest.php on line 8

Warning: Unable to find file identifier 0 in /usr/local/httpd/htdocs/regent/pdftest.php on line 9

Warning: Unable to find file identifier 0 in /usr/local/httpd/htdocs/regent/pdftest.php on line 10


My PHP-commandline is the following:
'./configure' '--prefix=/usr' '--bindir=/usr/bin' '--libdir=/usr/lib' '--with-exec-dir=%{libdir}/php/bin' '--with-mysql=/usr' '--with-gd=/usr' '--with-imap' '--with-imap-ssl=/usr' '--with-tiff-dir=/usr' '--with-jpeg-dir=/usr' '--with-png-dir=/usr' '--with-xpm-dir=/usr/X11R6' '--with-pdflib=yes' '--with-ldap=yes' '--with-zlib=yes' '--with-xml' '--with-ttf' '--with-readline' '--with-ftp' '--with-ndbm' '--with-gdbm' '--with-mcrypt=/usr/lib/libmcrypt' '--with-mm' '--with-interbase' '--enable-versioning' '--enable-yp' '--enable-bcmath' '--enable-trans-sid' '--enable-inline-optimization' '--enable-track-vars' '--enable-magic-quotes' '--enable-safe-mode' '--enable-sockets' '--enable-calendar' '--enable-sysvsem' '--enable-sysvshm' '--enable-shmop' '--enable-exif' '--enable-ftp' '--enable-memory-limit' '--enable-wddx' '--enable-filepro' '--enable-dbase' '--with-config-file-path=/etc/httpd' '--with-apxs=/usr/sbin/apxs' '--with-exec-dir=/usr/lib/apache/bin' 'i386-suse-linux-gnu'


If this is not a bug, a comment in the documentation could be very useful...

Greetings,

Christian G?king