Normally I wouldn't touch PHP due to the long history of security issues. Would this do anything to improve it? Love the idea of an embeddable interpreter.
If insecurities are part of PHP, no, not unless they deviate from the standard implementation. And there are most likely a bunch of undiscovered stack overflow type insecurities in this new interpreter as well.
If you want an embeddable interpreter, Tcl and Lua are other possibilities.
It might be less secure due to the fact that this is a new, untested and unproved implementation of a pretty quirky language.