Builder.php

<?php

/**
 * Class SQLBuilder
 */
class SQLBuilder
{
    private $from_table = null;
    /** @var array */
    private $select_columns = [];

    public function clear()
    {
        $this->from_table = null;
        $this->select_columns = [];

        return $this;
    }

    public function from($table_name)
    {
        $this->from_table = $table_name;

        return $this;
    }

    public function select($columns = '*')
    {
        if (is_string($columns)) {
            $columns = explode(',', $columns);
        }

        foreach ($columns as $column) {
            $column = trim($column);

            if ($column) {
                $this->select_columns[] = $column;
            }
        }

        return $this;
    }

    public function text()
    {
        /* SQLインジェクション... */
        return 'SELECT ' . $this->parse_select_columns() . ' FROM ' . $this->from_table;
    }

    private function parse_select_columns()
    {
        $select_columns = $this->select_columns;
        if (empty($select_columns)) {
            $select_columns = ['*'];
        }

        /* SQLインジェクション... */
        return implode(',', $select_columns);
    }
}

class MySQL_Connector
{
    /**
     * @param SQLBuilder $builder
     */
    public function execute($builder)
    {
        echo $builder->text();
    }
}

class User
{
    /** @var MySQL_Connector */
    private $connector;

    /** @var SQLBuilder $builder */
    private $builder;

    public function __construct()
    {
        $this->connector = new MySQL_Connector();
        $this->builder = new SQLBuilder();
    }

    public function get_user_name()
    {
        $this->builder = $this->builder->clear()->select(['user_name'])->from('users');
        $this->connector->execute($this->builder);
    }

    public function get_user()
    {
        $this->builder = $this->builder->clear()->from('users');

        $this->connector->execute($this->builder);
    }
}

echo " ------- \n";

$user = new User();
$user->get_user();
echo "\n ------- \n";

$user->get_user_name();
echo "\n ------- \n";