how can i create a tagging system using php and mysql?
You're describing a many-to-many relationship between Articles and Tags. You'd want to use an intermediate junction table to resolve that relationship.
Now, to find all articles that match a selected tag:
SELECT a.article_title FROM Tag t INNER JOIN Article_Tag_Xref atx ON t.tag_id = atx.tag_id INNER JOIN Article a ON atx.article_id = a.article_id WHERE t.tag_name = 'SelectedTagName'
tbl_articles has articleID, etc.
tbl_tags has tagID, etc.
tbl_taggedarticles has taggedArticleID, articleID, tagID
SELECT articleID FROM tbl_taggedarticles WHERE tagID = "searchedID"