Installing PHP can somewhat confusing when you dont know the right package, here is the step by step Php Installation on Ubuntu/Linux i get from www.php.net

1. Install right package
  #apt-get install php5-common libapache2-mod-php5 php5-cli

2. Restart apache to apply changes
# /etc/init.d/apache2 stop
# /etc/init.d/apache2 start

3. search for php-database module
# apt-cache search php5
# aptitude search php5
# aptitude search php5 |grep -i mysql

4. install php mysql and php curl module
# apt-get install php5-mysql php5-curl

source :
http://www.php.net/manual/en/install.unix.debian.php