grep -r --include=*.php "some_string" some_directory

Usage: grep [OPTION]... PATTERN [FILE] ...
Search for PATTERN in each FILE or standard input.
Example: grep -i 'hello world' menu.h main.c

Regexp selection and interpretation:
  -E, --extended-regexp     PATTERN is an extended regular expression
  -F, --fixed-strings       PATTERN is a set of newline-separated strings
  -G, --basic-regexp        PATTERN is a basic regular expression
  -e, --regexp=PATTERN      use PATTERN as a regular expression
  -f, --file=FILE           obtain PATTERN from FILE
  -i, --ignore-case         ignore case distinctions
  -w, --word-regexp         force PATTERN to match only whole words
  -x, --line-regexp         force PATTERN to match only whole lines
  -z, --null-data           a data line ends in 0 byte, not newline

Miscellaneous:
  -s, --no-messages         suppress error messages
  -v, --invert-match        select non-matching lines
  -V, --version             print version information and exit
      --help                display this help and exit
  -Z, --decompress          decompress input before searching (HAVE_LIBZ=1)
      --mmap                use memory-mapped input if possible

Output control:
  -b, --byte-offset         print the byte offset with output lines
  -n, --line-number         print line number with output lines
  -H, --with-filename       print the filename for each match
  -h, --no-filename         suppress the prefixing filename on output
  -q, --quiet, --silent     suppress all normal output
      --binary-files=TYPE   assume that binary files are TYPE
                            TYPE is 'binary', 'text', or 'without-match'.
  -a, --text                equivalent to --binary-files=text
  -I                        equivalent to --binary-files=without-match
  -d, --directories=ACTION  how to handle directories
                            ACTION is 'read', 'recurse', or 'skip'.
  -r, --recursive           equivalent to --directories=recurse.
  -L, --files-without-match only print FILE names containing no match
  -l, --files-with-matches  only print FILE names containing matches
  -c, --count               only print a count of matching lines per FILE
      --null                print 0 byte after FILE name

Context control:
  -B, --before-context=NUM  print NUM lines of leading context
  -A, --after-context=NUM   print NUM lines of trailing context
  -C, --context[=NUM]       print NUM (default 2) lines of output context
                            unless overridden by -A or -B
  -NUM                      same as --context=NUM
  -U, --binary              do not strip CR characters at EOL (MSDOS)
  -u, --unix-byte-offsets   report offsets as if CRs were not there (MSDOS)

`egrep' means `grep -E'.  `fgrep' means `grep -F'.
With no FILE, or when FILE is -, read standard input.  If less than
two FILEs given, assume -h.  Exit status is 0 if match, 1 if no match,
and 2 if trouble.

Report bugs to <bug-gnu-utils@gnu.org>.

grep (GNU grep) 2.4d

Copyright (C) 1988, 1992-1998, 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

grep version 0.9
/usr/local/bin/grepusage: grep [-[AB] num] [-CEFGHLPRSVZabchilnoqsvwx] [-e patttern] [-f file]

find . -name "*.php" -exec grep -H "some_string" {} \; 

grep "some_string" `find some_folder -name "*.some_extension"`

grep "some_string" `find some_folder -name "*.some_extension"`

/usr/bin/grep: Argument list too long.

find some_dir -name "*.some_ext" | xargs -l10 grep "some_expr"   

find some_dir -name "*.some_ext" | xargs -l10 grep "some_expr"   

find some_dir -name "*.some_ext" | xargs -L10 grep "some_expr"