It should be obvious that this should only be used if you're making one write, if you are writing multiple times to the same file you should handle it yourself with fopen and fwrite, the fclose when you are done writing.
Benchmark below:
file_put_contents() for 1,000,000 writes - average of 3 benchmarks:
real 0m3.932s
user 0m2.487s
sys 0m1.437s
fopen() fwrite() for 1,000,000 writes, fclose() - average of 3 benchmarks:
real 0m2.265s
user 0m1.819s
sys 0m0.445s