I am doing the following in PHP:
exec(\'java -jar \"/opt/flex3/lib/mxmlc.jar\" +flexlib \"/opt/flex3/frameworks\" MyAS3App.as -default-size 360 280 -output M
Turns out it was a bug specific to the PHP stack MAMP (http://www.mamp.info/).
Turns out any invocation of the JVM following fails under MAMP, e.g.:
exec('java -version');
The fix is to prefix the command with
export DYLD_LIBRARY_PATH="";
Also I realized there's no reason to use that method of invoking mxmlc.
So here's the final, working command:
exec('export DYLD_LIBRARY_PATH=""; mxmlc MyAS3App.as -default-size 360 280 -output MyAS3App.swf');