I'm going to recommend the same method I recommended to Ani_BEE
for hidden joints. It worked really well with my Freya spear, which is like...8-9 ft tall. My only comment is if your spear has a heavy head to it, be aware that PVC piping WILL bend visibly if you don't reinforce it with a wooden dowel inside.
If your prop has convenient joints, you can use the PVC connector suggestion the above mentioned thread...but DO pick up PVC glue for a strong bond. (and use the glue outside, and leave it there for a while, because the fumes are TOXIC).