azure-storage-common/src/Common/SharedAccessSignatureHelper.php
protected function validateAndSanitizeSignedService($signedService)
    {
        // validate signed service is not null or empty
        Validate::canCastAsString($signedService, 'signedService');
        Validate::notNullOrEmpty($signedService, 'signedService');

        // The signed service should only be a combination of the letters b(lob) q(ueue) t(able) or f(ile)
        $validServices = ['b', 'q', 't', 'f'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedService),
            $validServices
        );
    }
protected function validateAndSanitizeSignedService($signedService)
    {
        // validate signed service is not null or empty
        Validate::canCastAsString($signedService, 'signedService');
        Validate::notNullOrEmpty($signedService, 'signedService');

        // The signed service should only be a combination of the letters b(lob) q(ueue) t(able) or f(ile)
        $validServices = ['b', 'q', 't', 'f'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedService),
            $validServices
        );
    }
Validates and sanitizes the signed service parameter

@param string $signedService Specifies the signed services accessible
with the account SAS.

@return string