php man page for which

WHICH(1)						      General Commands Manual							  WHICH(1)

NAME
       which - locate a command

SYNOPSIS
       which [-a] filename ...

DESCRIPTION
       which  returns  the  pathnames  of the files (or links) which would be executed in the current environment, had its arguments been given as
       commands in a strictly POSIX-conformant shell.  It does this by searching the PATH for executable files matching the  names  of	the  argu-
       ments. It does not canonicalize path names.

OPTIONS
       -a     print all matching pathnames of each argument

EXIT STATUS
       0      if all specified commands are found and executable

       1      if one or more specified commands is nonexistent or not executable

       2      if an invalid option is specified

Debian								    29 Jun 2016 							  WHICH(1)