(PECL stats >= 1.0.0)
stats_standard_deviation — Returns the standard deviation
$a
   [, bool $sample = false
  ] )
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 standard deviation on success; FALSE on failure.
  
   Raises an E_WARNING when there are fewer than 2 
   values in a.