How to downgrade PHP from 5.5.9 to 5.4 in Ubuntu 14.04 (Installed with LAMP)

First you need to remove all php5 files :

sudo apt-get remove --purge `dpkg -l | grep php | grep -w 5.5 | awk '{print $2}' | xargs`

After that try to search for php 5.4. available resources :

apt-cache policy php5

My result :

php5:
  Installed: (none)
  Candidate: 5.5.9+dfsg-1ubuntu4.4
  Version table:
     5.5.9+dfsg-1ubuntu4.4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     5.5.9+dfsg-1ubuntu4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

If there is no resources available than we need to add some by following command :

echo "deb http://php53.dotdeb.org stable all" | sudo tee -a /etc/apt/sources.list

and

apt-get update

If there is a problem with signatures like following :

Reading package lists... Done
W: GPG error: http://php53.dotdeb.org stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY xxxxxCODExxxx

we need to sign our Deb repository by commands :

gpg --keyserver keys.gnupg.net --recv-key xxxxxCODExxxx
gpg -a --export xxxxxCODExxxx | sudo apt-key add - apt-get update

After that execute :

sudo apt-get update
apt-cache policy php5

you should see last result :

php5:
  Installed: (none)
  Candidate: 5.5.9+dfsg-1ubuntu4.4
  Version table:
     5.5.9+dfsg-1ubuntu4.4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     5.5.9+dfsg-1ubuntu4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
     5.4.33-1~dotdeb.1 0
        500 http://php53.dotdeb.org/ stable/all amd64 Packages

and finally install your version by hitting :

apt-get install php5=5.4.33-1~dotdeb.1

Enjoy ;)

add dotdeb repo by following command :

echo "deb http://php53.dotdeb.org stable all" | sudo tee -a /etc/apt/sources.list

Fetch and install the GnuPG key

wget http://www.dotdeb.org/dotdeb.gpg
sudo apt-key add dotdeb.gpg

Then

sudo apt-get update

The easy way to install the specific version is via synaptic

all done

also you should downgrade the apache to install right "libapache2-mod-php5"

add precise repo by the following command

echo "deb http://security.ubuntu.com/ubuntu precise-security main" | sudo tee -a /etc/apt/sources.list && sudo apt-get update

then

sudo aptitude install apache2=2.2.22-1ubuntu1.7

make sure you accept the following

The following actions will resolve these dependencies:

Install the following packages:
1) apache2-mpm-prefork [2.2.22-1ubuntu1.7 (precise-security)]
2) apache2.2-bin [2.2.22-1ubuntu1.7 (precise-security)]

then

sudo aptitude install libapache2-mod-php5=5.4.36-1~dotdeb.1

don't forget to set specific version when try to install an php package

sudo aptitude install php5-mysql=5.4.36-1~dotdeb.1