need to generate fixed-length file

I'm having trouble generating a fixed-length (i.e., non-delimited) file format out of PHP 4, based on queries out of MySQL. These files are getting imported into a Unidata database and require a very specific byte-mapping format. However, while the fopen used with length will TRUNCATE to a specific length, it also won't pad to that length. Is there a function I'm missing somewhere that will let you specify a field length and pad accordingly? If not, any suggestions as to approach will be greatly appreciated! Many thanks, Karen Goeller