what was the last value of a phosphate laboratory testing of patient 23002 since 05/2105?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 23002 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'phosphate' ) and strftime('%y-%m',labevents.charttime) >= '2105-05' order by labevents.charttime desc limit 1