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

        // The signed resource type should only be a combination of the letters s(ervice) c(container) or o(bject)
        $validResourceTypes = ['s', 'c', 'o'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedResourceType),
            $validResourceTypes
        );
    }
protected function validateAndSanitizeSignedResourceType($signedResourceType)
    {
        // validate signed resource type is not null or empty
        Validate::canCastAsString($signedResourceType, 'signedResourceType');
        Validate::notNullOrEmpty($signedResourceType, 'signedResourceType');

        // The signed resource type should only be a combination of the letters s(ervice) c(container) or o(bject)
        $validResourceTypes = ['s', 'c', 'o'];

        return $this->validateAndSanitizeStringWithArray(
            strtolower($signedResourceType),
            $validResourceTypes
        );
    }
Validates and sanitizes the signed resource type parameter

@param string $signedResourceType    Specifies the signed resource types
that are accessible with the account
SAS.

@return string