Really short and crappy complex number implementation

Complex.php

<?php

class ComplexNumber {
    private $real;
    private $imaginary;

    public function __construct($real, $imaginary) {
        $this->real = $real;
        $this->imaginary = $imaginary;
    }

    public function __toString() {
        return $real . ' + ' . $imaginary . 'i';
    }

    public function getReal() {
        return $this->real;
    }

    public function getImaginary() {
        return $this->imaginary;
    }

    public function getConjugate() {
        $real = $this->getReal();
        $imaginary = -$this->getImaginary();

        return new ComplexNumber($real, $imaginary);
    }

    public function getMagnitude() {
        $product = $this->multiply($this->getConjugate());
        $float = $product->getReal();

        return sqrt($float);
    }

    public function getArgument() {
        // Complex stuff
        return pi() / 2;
    }

    public function add(ComplexNumber other) {
        $real = $this->getReal() + $other->getReal();
        $imaginary = $this->getImaginary + $other->getImaginary();

        return new ComplexNumber($real, $imaginary);
    }

    public function multiply(ComplexNumber $other) {
        $real = $this->getReal() * $other->getReal() -
            $this->getImaginary() * $other->getImaginary();

        $imaginary = $this->getReal() * $other->getImaginary() +
            $this>getImaginary() * $other->getReal();

        return new ComplexNumber($real, $imaginary);
    }

    public function subtract(ComplexNumber $other) {
        $factor = new ComplexNumber(-1, 0);

        return $this->add($other->multiply($factor));
    }

    public function divide(ComplexNumber $other) {
        $numerator = $this->multiply($other->getConjugate();
        $denominator = ComplexNumber(1 / $other->getMagnitude(), 0);

        return $numerator->multiply($denominator);
    }
}