I have oracle server running on this node in $ORACLE_HOME and then I put all the instant client (basic and SDK) libraries in /instantclient_10_2 directory and my LD_LIBRARY_PATH points to /instantclient_10_2:$ORACLE_HOME/lib
So that means you have two oracle products on one machine. So that probably means your PHP (which is using instant client) gets ORA-12154.
And when you do test case using sqlplus (which is probably using Oracle server files/libraries and settings) it is working.
So there are several options:
1. you need to copy tnsnames.ora (from server install) and put it to one of expected (by instant client) locations: $HOME/.tnsnames.ora (Note Apache by default runs under user "nobody". Of course don't change it to "root"!), /etc/tnsnames.ora / $ORACLE_HOME/network/admin/tnsnames.ora (in your case /instantclient_10_2/network/admin/tnsnames.ora)
2. OR you need to set TNS_ADMIN variable with path to Oracle Server Home/network/admin directory. In this case your existing tnsnames.ora will be used for resolution.