If you install php-ldap module with the sentence:

sudo apt-get install php5-ldap

and you get the error:

The following NEW packages will be installed:
  php5-ldap
0 upgraded, 1 newly installed, 0 to remove and 104 not upgraded.
Need to get 20.7 kB of archives.
After this operation, 94.2 kB of additional disk space will be used.
Err http://es.archive.ubuntu.com/ubuntu/ quantal-updates/main php5-ldap i386 5.4.6-1ubuntu1.2
  404  Not Found
Err http://security.ubuntu.com/ubuntu/ quantal-security/main php5-ldap i386 5.4.6-1ubuntu1.2
  404  Not Found [IP: 91.189.91.14 80]
Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/p/php5/php5-ldap_5.4.6-1ubuntu1.2_i386.deb  404  Not Found [IP: 91.189.91.14 80]
E: Unable to fetch some archives, maybe run apt-get update or try with –fix-missing?

you must execute:

sudo apt-get update

and try the installation again.

Enjoy!!!