Core/Toolbox/Arrays/Flatten.php
public function setGlue(string $glue)
    {
        $allowed_glues = [
            '.',
            '_',
            '-'
        ];

        if (!in_array($glue, $allowed_glues)) {
            Throw new ArraysException('This glues "%s" is not permitted. Allowed glues are "%s"', $glue, implode(', ', $allowed_glues));
        }

        $this->glue = $glue;
    }
public function setGlue(string $glue)
    {
        $allowed_glues = [
            '.',
            '_',
            '-'
        ];

        if (!in_array($glue, $allowed_glues)) {
            Throw new ArraysException('This glues "%s" is not permitted. Allowed glues are "%s"', $glue, implode(', ', $allowed_glues));
        }

        $this->glue = $glue;
    }
Sets glue

@param string $glue