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I'm using filethingie to upload files to my website. When i upload files it gaves following error.

[07-Feb-2013 08:52:28 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:55:00 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:55:01 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:57:16 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:57:16 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:19 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:19 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:28 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:29 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:52 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 08:58:55 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 09:02:19 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 09:02:25 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 09:02:29 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 09:02:54 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0
[07-Feb-2013 09:02:59 UTC] PHP Warning:  preg_match() [<a href='function.preg-match'>function.preg-match</a>]: Unknown modifier '\' in /home/softlogi/public_html/Softlogic_Research/ft-admin.php on line 113
[07-Feb-2013 09:03:06 UTC] PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20090626/pdf.so: cannot open shared object file: No such file or directory in Unknown on line 0

And displays

name.pdf was not uploaded. File type not allowed

Why is this issue occoers and how can i fix it?

It looks like you've added 'pdf.so' to your php.ini, without actually installing the extension.

PHP Startup: Unable to load dynamic library

Looks like pdf extension is not installed correctly. You can read the install section of the plugin to fix this error. Then read about how to use the plugin. Just follow the instruction and you will be fine.