[PHP7] Simpler Passwortgenerator

<?php
    namespace FidgetSpinner2004\PasswordGenerator;

    interface IRandomNumberGenerator
    {
        // inclusive min, exclusive max
        public function getRandomNumberInRange($min, $max);
    }

    class RandRandomNumberGenerator implements IRandomNumberGenerator
    {
        public function getRandomNumberInRange($min, $max)
        {
            if($max <= $min)
            {
                return new InvalidArgumentException('$max must be larger than $min');
            }

            return $min + (rand() % $max);
        }
    }

    interface IPasswordBuffer
    {
        public function appendString($string);
        public function toString();
        public function reset();
    }

    class StringPasswordBuffer implements IPasswordBuffer
    {
        private $buffer;

        public function __construct()
        {
            $buffer = '';
        }

        public function appendString($string)
        {
            $this->buffer .= $string;
        }

        public function toString()
        {
            return $this->buffer;
        }

        public function reset()
        {
            $this->buffer = '';
        }
    }

    interface IStringCollection
    {
        public function getStringByIndex($index);
        public function addString($string);
        public function addStringsFromArray(Array $stringArray);
        public function getSize();
    }

    class ArrayStringCollection implements IStringCollection
    {
        private $strings;

        public function __construct()
        {
            $this->strings = array();
        }

        public function getStringByIndex($index)
        {
            return $this->strings[$index];
        }

        public function addString($string)
        {
            array_push($this->strings, $string);
        }

        public function addStringsFromArray(Array $stringArray)
        {
            foreach($stringArray as $string)
            {
                $this->addString($string);
            }
        }

        public function getSize()
        {
            return count($this->strings);
        }
    }

    interface IPasswordSchema
    {
        public function getLength();
        public function getStringCollection();
    }

    class StandardPasswordSchema implements IPasswordSchema
    {
        private $length;
        private $stringCollection;

        public function __construct($length, IStringCollection $stringCollection) {
            $this->length = $length;
            $this->stringCollection = $stringCollection;
        }

        public function getLength()
        {
            return $this->length;
        }

        public function getStringCollection()
        {
            return $this->stringCollection;
        }
    }

    interface IPasswordGenerator
    {
        public function generatePassword(IPasswordBuffer $buffer, IRandomNumberGenerator $rng);
    }

    class StaticSchemaPasswordGenerator implements IPasswordGenerator
    {
        private $schema;

        public function __construct(IPasswordSchema $schema)
        {
            $this->schema = $schema;
        }

        public function generatePassword(IPasswordBuffer $buffer, IRandomNumberGenerator $rng)
        {
            $chars = $this->schema->getStringCollection();
            $len = $this->schema->getLength();
        
            for($i = 0; $i < $len; $i++)
            {
                $randomString = $chars->getStringByIndex($rng->getRandomNumberInRange(0, $chars->getSize()));
                $buffer->appendString($randomString);
            }
        }

        public function getSchema()
        {
            return $this->schema;
        }
    }
?>

<?php
    require('PasswordGenerator.php');
    use FidgetSpinner2004\PasswordGenerator;

    $strCollection = new PasswordGenerator\ArrayStringCollection();
    $strCollection->addStringsFromArray(["a", "b", "c", "d", "e", "f", "0", "1", "2", "3"]);
    $schema = new PasswordGenerator\StandardPasswordSchema(/* $length = */24, $strCollection);
    $gen = new PasswordGenerator\StaticSchemaPasswordGenerator($schema);
    $buffer = new PasswordGenerator\StringPasswordBuffer();
    $rng = new PasswordGenerator\RandRandomNumberGenerator();
    $gen->generatePassword($buffer, $rng);
    echo $buffer->toString();
?>