pwede yan, gamit ka nung javascript canvas libraries like paper.js, kinetic.js etc., tapos dapat nkaready na ung mga tshirt background mo and mga sample images na idadrag nlng nila then pwede ka mag add edit delete ng text , and manipulate nila ung buong items inside the canvas using the said canvas libraries. then i output mo as one image ung design.
sa PHP, just use wordpress para may layout ung site mo and for adding updates., then isingit mo sa isa sa mga pages ung software mo that will use any of the canvas libraries
pagaralan mo nlng marami tutorial sa online, search mo sa google. goodluck
web developer here btw, anyone who needs a website, i can help for a price.PM ME