It interesting to note that the expected behavior for this function may change according to your ICU version.
In ICU 4.4.2 (standard for Ubuntu 10.* with PHP 5.3.5)
With locale 'en', input of 100,1 returns 1001
In ICU 4.8.1 (standard for Ubuntu 12.* with PHP 5.3.10)
With locale 'en', input of 100,1 returns "false"
Be sure to note your ICU version in phpinfo() to be sure you will get the expected output.