Internal Server Error 500 – website not showing

Hi,
my website is down, displaying an Internal Server Error:
“The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@”my-domain”.co.uk and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Apache/2.2.29 (Unix) mod_ssl/2.2.29 OpenSSL/1.0.1e-fips mod_bwlimited/1.4 Server at “my-domain” Port 80″

I have tried the following with no success:
– Renaming .htaccess
– Deactivating plugins
– re-installing WordPress via FTP
– Checking file permissions (all seem correct)
– Increasing PHP memory limit via wp-config.php

Example error logs look like this:
[Sat Mar 07 08:54:10 2015] [error] [client 78.105.7.19] Premature end of script headers: index.php, referer: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2F”my-domain”%2F&ei=mrz6VIrdNYq07gb7pYHwCg&usg=AFQjCNFxZxu_dNafdSWmzjMWAErge1Vvjg&bvm=bv.87611401,d.ZGU [Sat Mar 07 08:54:45 2015] [error] [client 78.105.7.19] Failed loading /usr/local/Zend/lib/ZendExtensionManager.so: /usr/local/Zend/lib/ZendExtensionManager.so: cannot open shared object file: No such file or directory, referer: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQoAIwAg&url=http%3A%2F%2F”my-domain”%2F&ei=wLz6VNDpMMSd7gar2ICABg&usg=AFQjCNEgi1or2xXq7O82_orbAjvIox0kVg&bvm=bv.87611401,d.ZGU [Sat Mar 07 08:54:45 2015] [error] [client 78.105.7.19] Premature end of script headers: index.php, referer: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQoAIwAg&url=http%3A%2F%2F”my-domain”%2F&ei=wLz6VNDpMMSd7gar2ICABg&usg=AFQjCNEgi1or2xXq7O82_orbAjvIox0kVg&bvm=bv.87611401,d.ZGU [Sat Mar 07 08:54:45 2015] [error] [client 78.105.7.19] Failed loading /usr/local/Zend/lib/ZendExtensionManager.so: /usr/local/Zend/lib/ZendExtensionManager.so: cannot open shared object file: No such file or directory, referer: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQoAIwAg&url=http%3A%2F%2F”my-domain”%2F&ei=wLz6VNDpMMSd7gar2ICABg&usg=AFQjCNEgi1or2xXq7O82_orbAjvIox0kVg&bvm=bv.87611401,d.ZGU [Sat Mar 07 08:54:45 2015] [error] [client 78.105.7.19] Premature end of script headers: index.php, referer: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQoAIwAg&url=http%3A%2F%2F”my-domain”%2F&ei=wLz6VNDpMMSd7gar2ICABg&usg=AFQjCNEgi1or2xXq7O82_orbAjvIox0kVg&bvm=bv.87611401,d.ZGU

Can anyone help please or suggest where I go from here? Thanks!