The find command is used to search for files in directories, and possibly to perform actions on the found files.

find . -name "*.txt"

Search for files with the .txt extension in the current directory and subdirectories.

find / -name "*.php"

Search for files with the .php extension from the root.

find / -name "*.php" 2>/dev/null

The screen is saturated with the list of directories that are forbidden to us to read. The command 2>/dev/null redirects the errors to the virtual file /dev/null which makes them disappear from the display.

find / -name "*.php" -exec ls {} \; 2>/dev/null

The -exec option is used to run a command on each file found. Often ls -al, or cat.
{} is replaced by the name of the file found.
\; is put at the end of the command to be executed.