How PHP practically used in coding?

1. Constant
   1. define('ROOTURI', get_template_directory_uri() );
   2. Automatically global.
2. basename()
1. File or last folder name of a path
2. echo basename(ROOTURI);
1. myTheme.
3. dirname()
1. Directory name of a path.
4. include ()
1. $mobile_path =plugin.php';
   if ( file_exists( $mobile_path ) ) { include( $mobile_path ); }
2. if ( file_exists( ROOTDIR . '/a/setup.php' ) ) include( ROOTDIR . '/a/setup.php' );
5. stripos(string,find,start)
1. Position number of the first occurrence inside the string: