azure-storage-common/src/Common/SharedAccessSignatureHelper.php
protected function validateAndSanitizeSignedPermissions(
        $signedPermissions
    ) {
        // validate signed permissions are not null or empty
        Validate::canCastAsString($signedPermissions, 'signedPermissions');
        Validate::notNullOrEmpty($signedPermissions, 'signedPermissions');

        $validPermissions = ['r', 'w', 'd', 'l', 'a', 'c', 'u', 'p'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedPermissions),
            $validPermissions
        );
    }
protected function validateAndSanitizeSignedPermissions(
        $signedPermissions
    ) {
        // validate signed permissions are not null or empty
        Validate::canCastAsString($signedPermissions, 'signedPermissions');
        Validate::notNullOrEmpty($signedPermissions, 'signedPermissions');

        $validPermissions = ['r', 'w', 'd', 'l', 'a', 'c', 'u', 'p'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedPermissions),
            $validPermissions
        );
    }
Validates and sanitizes the signed permissions parameter

@param string $signedPermissions Specifies the signed permissions for the
account SAS.

@return string