In order to match the output of this function with Excel's TDIST function, you must take 1 - the value.  For example, for a two-tailed T-distribution for x=4 and degrees of freedom=2, the formula would be:

(1 - stats_cdf_t(4, 2, 1)) * 2

For a single tail, do not include the multiply by 2 portion.

Input  and return values of stats_cdf_t depends on the $which-parameter:
<?php
function get_t_p($t, $df){
  return stats_cdf_t($t, $df, 1);
}
function get_t_t($p, $df){
  return stats_cdf_t($p, $df, 2);
}
function get_t_df($p, $t){
  return stats_cdf_t($p, $t, 3);
}
?>