although the specs (both php & sphinx manuals) states that field weight should be positive integer, values lower than 1 (0 for example) would not work. Found this on a blog and i hope this will help you ...
(PECL sphinx >= 0.1.0)
SphinxClient::setFieldWeights — Set field weights
Binds per-field weights by name.
Match ranking can be affected by per-field weights. See » Sphinx documentation for an explanation on how phrase proximity ranking is affected. This call lets you specify non-default weights for full-text fields.
The weights must be positive 32-bit integers, so be careful not to hit 32-bit integer maximum. The final weight is a 32-bit integer too. Default weight value is 1. Unknown field names are silently ignored.
Associative array of field names and field weights.
TRUE on success or
FALSE on failure.