/etc/apt/sources.list

deb http://ftp.debian.org/debian/ squeeze main contrib non-free
deb http://security.debian.org/ squeeze/updates main contrib non-free

apt-get update

apt-cache policy php5
php5:

Installed: 5.4.39-0+deb7u2
Package pin: 5.4.39-0+deb7u2
Candidate: 5.3.3-7+squeeze19

apt-get install –reinstall libapache2-mod-php5=5.3.3-7+squeeze19 php-pear=5.3.3-7+squeeze19 php5=5.3.3-7+squeeze19 php5-cli=5.3.3-7+squeeze19 php5-common=5.3.3-7+squeeze19 php5-curl=5.3.3-7+squeeze19 php5-gd=5.3.3-7+squeeze19 php5-imap=5.3.3-7+squeeze19 php5-intl=5.3.3-7+squeeze19 php5-mcrypt=5.3.3-7+squeeze19 php5-mysql=5.3.3-7+squeeze19 php5-pspell=5.3.3-7+squeeze19 php5-xmlrpc=5.3.3-7+squeeze19 php5-xsl=5.3.3-7+squeeze19