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 em caso de sucesso ou
FALSE em caso de falha.