Try something like grep -r -n -i --include="*.html *.php *.js" searchstrinhere .
the -i
makes it case insensitlve
the .
at the end means you want to start from your current directory, this could be substituted with any directory.
the -r
means do this recursively, right down the directory tree
the -n
prints the line number for matches.
the --include
lets you add file names, extensions. Wildcards accepted
For more info see: http://www.gnu.org/software/grep/