PHP: Round a number down to two decimal places

RoundDown2DemialPlaces.php

    /**
     * Custom function to round down to the nearest lowest number at 2 decimal places.
     * e.g: 1.006 rounds down to 1.00
     * Warning: Do not use PHP_ROUND_HALF_DOWN;
     *      round(0.006, 2, PHP_ROUND_HALF_DOWN) does not round down to 0
     *
     * @param $value
     * - Must be numeric
     * - Must be a positive value
     * @return float
     * @throws \Exception
     */
    static function roundDown2DecimalPlaces($value)
    {
        // Error Check
        // - Must be numeric
        // - Must be a positive value
        if(!is_numeric($value) || (is_numeric($value) && $value < 0)) {
            throw new \Exception('Value must be numeric. Got type: '.gettype($value));
        }

        $strValue = strval($value);
        // Parts: dollars and cents
        $parts = explode('.', $strValue);
        if(count($parts) == 2) {
            // make sure cents is one or two characters at most
            $cents = substr($parts[1], 0, 2);
            // cents as a string now
            // should be two characters long
            // if not, pad zeros to the end
            $cents = str_pad($cents, 2, '0', STR_PAD_RIGHT);
        }
        else {
            $cents = '00';
        }
        $s = $parts[0].'.'.$cents;
        return $s;
    }

// -----------------------------------------------------------------
/*
  PHPUnit Test
  GeneralHelperTest.php functions
 */
    public function test_roundDown2DecimalPlaces()
    {
        $i = 200;
        while($i < 201.1) {

            $this->_roundDown2DecimalPlaces($i);

            // increment
            $i += 0.001;
        }

        $i = 0;
        while($i < 11) {

            $this->_roundDown2DecimalPlaces($i);

            // increment
            $i += 0.5;
        }

        $i = 135.45;
        echo "\n\nPass #1 - round $i";
        $r = $this->_roundDown2DecimalPlaces($i);
        self::assertEquals(135.45, $r, 'Failed this->_roundDown2DecimalPlaces !!!!!!!!');

        echo "\n\nPass #2";
        echo "\n\nRound a rounded value...";
        $r = GeneralHelper::roundDown2DecimalPlaces($r);
        self::assertEquals(135.45, $r, 'Failed GeneralHelper::roundDown2DecimalPlaces');

    }

    /**
     * Keep this here, once proved working methods, we can copy a method to the Library Function.
     *
     * @param $i
     */
    private function _roundDown2DecimalPlaces($i)
    {
        $rounded = GeneralHelper::roundDown2DecimalPlaces($i);
        echo "\n$i\t | rounded = $rounded";

        // Check
        $si = strval($i);
        $siLen = strlen($si);
        $roundedLen = strlen($rounded);
        if($siLen < $roundedLen) {
            // $si shorter, fix...
            // Contains Cents?
            if(strpos($si, '.') !== false) {
                // yes
                $check = str_pad($si, 2, '0');
            }
            else {
                // no
                $check = $si.'.00';
            }
        }
        elseif($siLen > $roundedLen) {
            // make them the same size
            $check = substr($si, 0, $roundedLen);
        }
        else {
            $check = $si;
        }

        // Check Equals
        self::assertEquals($check, $rounded, "Check equals failed: si = $si | check = $check | rounded = $rounded");
        // Check Cents is 2 places
        $ex = explode('.', $rounded);
        self::assertEquals(2, count($ex), "Rounded value does not have cents! $rounded");
        if(count($ex) == 2) {
            self::assertEquals(2, strlen($ex[1]), "Rounded value does not contain 2 decimal places! $rounded");
        }

        return $rounded;
    }