How to find file with name="php.ini" on linux using grep command

You would normally use find not grep to find files by name.

find / -name php.ini

If you must use grep

cd /; ls -lR | grep php.ini

In both cases replace "/" with the absolute or relative path for the directory you wish to start the search in.

Note that linux also has a locate command that relies on indexing - check it's man page for details. This is fastest if the right locations are indexed.

Just to add some more information...

find / -name php.ini
cd /; ls -lR  | grep php.ini

of course do work, but you could be better served with

locate php.ini

which uses the indexed filesystem database to locate the file. It is considerably faster. To update the filesystem index, the command is:

updatedb

These however required root as far as I remember.

disclaimer: I have not used linux for years for anything meaningful... I learnt these back then when the predominant distribution was slackware, and debian potato was not even out...