Problem: Items have optional price. If the price is 0 (or I can change it to null if it helps), then when displaying documents, those documents need to be sorted last when sorting by price. After long searching I think setMissingLast would help most, but I don't see any way of setting setMissingLast option in PECL Solr (and we can't change a search library just for this thing).
I also tried without success to use map(price,0,0,999) function query (only when sorting ascending).
How can I set setMissingLast for price field in PECL Solr, or are there any other solutions for making sure missing price documents are always sorted last?
I think you mean sortMissingLast. You don't need your library to support it, you just have to add sortmissingLast=true in your fieldType definition within the solr schema.