yii\db\ColumnSchemaBuilder::buildDefaultString PHP Method

    protected function buildDefaultString()
    {
        if ($this->default === null) {
            return $this->isNotNull === false ? ' DEFAULT NULL' : '';
        }
        $string = ' DEFAULT ';
        switch (gettype($this->default)) {
            case 'integer':
                $string .= (string) $this->default;
                break;
            case 'double':
                // ensure type cast always has . as decimal separator in all locales
                $string .= str_replace(',', '.', (string) $this->default);
                break;
            case 'boolean':
                $string .= $this->default ? 'TRUE' : 'FALSE';
                break;
            case 'object':
                $string .= (string) $this->default;
                break;
            default:
                $string .= "'{$this->default}'";
        }
        return $string;
    }