I think copy("src_file", "dest_file") should work on php4 for cache...

Also file() is the php4 version of file_get_contents() only that the output is not put into a string - php.net manual for more details/limitations...

Note this:
file_put_contents
Write a string to a file (PHP 5)
int file_put_contents ( string filename, mixed data [, int flags [, resource context]] )
This function is identical to calling fopen(), fwrite() and fclose() successively to write data to a file.