Return number of arithmetic slice in a given set - Solution to a problem on CODILITY

Arithmetic Slices.php

//Return number of arithmetic slices in a given set
    public function numberOfArithmeticSlicesFromSet(array $A)
    {
        $numbersCount = count($A);

        //mininum length of arithmetic slice is 3
		if($numbersCount < 3)
            return 0;

		$startOfSlice = 0;

		$numberOfSlices = 0;

		while($startOfSlice < $numbersCount-2) {

            $endOfSlice = $startOfSlice+1;

			$arithmeticDifference = $A[$endOfSlice] - $A[$startOfSlice];

			//continue in set until condition fails
			while($endOfSlice < $numbersCount-1 && $A[$endOfSlice+1] - $A[$endOfSlice] == $arithmeticDifference) {
                $endOfSlice++;
            }

            //this is the length of the found arightmetic slice
			$lengthOfSlice = $endOfSlice - $startOfSlice + 1;

			//at least 3 number must be in any valid slice
			if($lengthOfSlice >= 3){

                $numberOfSlices += (($lengthOfSlice-2)*($lengthOfSlice-1)/2);
            }

			$startOfSlice = $endOfSlice;
		}

		//sanity check
		return ($numberOfSlices > 1000000000) ? -1 : $numberOfSlices;
	}