To be clearer, as Cweiske pointed out, Jenkins fails the build because it cannot find the correct phing executable, as it looks for it in the wrong path (the one in the workspace).
Anyway, Jenkins search for that executable in other paths too, as the one you cited: "in my local PATH as I can invoke the phing command anywhere".
The problem with Your Local Path is that it is the YOUR and not the Jenkin's one. As Jenkins act on your Mac with its own user (jenkins, created during the installation process), so it cannot use Your Local Path.
Understood the problem, there is more than one possible solution (and i've tried them one by one with no success!), but the simpler (and the one that finally works!) is the "symlinking".
A note: please, take care of the difference between various types of "links" on Mac: SymLinks are different and not equals to Aliases or to Hard Links (http://macs.about.com/od/faq1/f/What-Are-Aliases-Symbolic-Links-And-Hard-Links-In-Mac-Os-X.htm).
Coming back, as in the usr/bin folder there are executables that each user on the mac can use, let's create a symlink into this folder to the executable you use every time, the one included in the folder in Your Local Path.
From Terminal, go to the usr/bin folder. Use the command "cd" to change the current directory and to go to the top of the root use something like:
> cd ../
With the command "pwd" you can see the complete path to the directory in which you currently are, and with the "ls" command you can list all the files and folders in it: use them to guide yourself in the folders tree.
So, assuming you already are in the usr/bin folder and assuming you are using MAMP as webserver, the command is:
> ln /Applications/MAMP/bin/php/php5.5.10/bin/phing ./phing
As you can see, the first parameter is the path to the executable you want to symlink, and the second parameter is the path to the folder in which you want to create the symlink. Don't forget to append the name of your new symlink!
Once done, try again to build with Jenkins: all should work well now... i hope!
Some troubleshooting:
1) You created the symlink in the wrong folder: use "rm" to remove a file
2) You haven't the right permissions: use "sudo" to act as a super admin
> sudo ln /Applications/MAMP/bin/php/php5.5.10/bin/phing ./phing