$chunk = substr($string,0,rindex($string,' ',200));

##</code><code>##

Benchmark: running japhy, rindex, tye, each for at least 3 CPU seconds...
     japhy:  2 wallclock secs ( 3.00 usr +  0.00 sys =  3.00 CPU) @ 244478.67/s (n=733436)
    rindex:  3 wallclock secs ( 3.00 usr +  0.00 sys =  3.00 CPU) @ 842494.33/s (n=2527483)
       tye:  4 wallclock secs ( 3.39 usr +  0.00 sys =  3.39 CPU) @ 305243.95/s (n=1034777)