The source code actually has much more complete documentation (more functions, a better description of what they do, and parameters). You can see the latest here:
http://svn.php.net/viewvc/pecl/stats/trunk/
Notably, try clicking the latest revision number for php_stats.c
There are some undocumented functions in this extension, eg I found the following by reading the DCDFLIB docs and some trial and error:
<?php
/**
* stats_cdf_normal($x, $mean, $sd, 1)
* @return float cumulative probablity
*/
echo stats_cdf_normal(1.96, 0, 1, 1) . "\n";
/**
* stats_cdf_normal($p, $mean, $sd, 1)
* @return float x which bounds cumulative probalility $p
*/
echo stats_cdf_normal(0.975, 0, 1, 2) . "\n";
// less useful
// echo stats_cdf_normal(0.4,0.5,0.6,3) . "\n";
// echo stats_cdf_normal(0.4,0.5,0.6,4) . "\n";
?>