What PHP-5.5 Extensions are Required on Magento-1.x Server ?

Apart from php extensions that get installed with php installation itself, following extensions are needed with php5.5 on Magento 1.x server.

php5-curl php5-gd php5-mcrypt

Along with these, you have to install:

libapache2-mod-php5 libcurl3

Also set date.time zone and increase memory limit in php.ini :

date.timezone = Asia/Kolkata
memory_limit = 512M

Enable mcrypt module as:

php5enmod mcrypt

Now restart the apache2 server:

service apache2 restart