我有问题。我需要在php中拆分字符串变量

I have code which extracting keywords, this keywords with td/idf rating and other options are in $tags. Variable k-keyword consisting of this word. but if i want to print all of these keywords, this keywords look like: one long string, I need to have this keywords separated with " ", or ","... I have something like that in phpI have code which extracting keywords, this key