Well, leeme explain it:
1.Zend is TOO expensive for personal, however most ISP have zend loader pre-installed, and although the so-called "dezender" floating around, most of them can't restore the source completely.
2.phpShield is cheap solution, however you need to ask the ISP to install it's loader(it's mostly impossible).
Some alternative(ie. no loader required...etc)
SourceCop (US$30)
http://www.sourcecop.com/It doesn't require you to install any loader on your space, the protect strength is not good, people with php skill may possibly revert the source ,but it works.
ionCube Standalone Encoder (US$199)
http://www.ioncube.com/sa_encoder.phpThis one has better protection, and doesn't need external loader, evalution available, a few of my friends are using it because zend encoder is nearly $1000.