(PECL stats >= 1.0.0)
stats_variance — Returns the variance
$a, bool $sample = false): float
Returns the variance of the values in a.
aThe array of data to find the standard deviation for. Note that all values of the array will be cast to float.
sample
Indicates if a represents a sample of the
population; defaults to false.
Returns the variance on success; false on failure.
This will return surprising values for most users, since the default of sample=false is rather unusual when speaking about variance.
Point is:
When using stats_variance($foo,TRUE) the sum is divided by n-1 while the default just divides it by n. So when you expect your variance to be lower you probable wanted to divide by n-1, therefore should set the second parameter =TRUE.