A series of Arabic characters and symbols in a message proves to be difficult to process with the Messages application (and others). The latter may crash and even cause the iPhone to restart.
In the hands of developers, the most recent beta version of iOS 8.4 fixes this bug. The stable version of iOS 8.4 is expected by June 30. It should not be ruled out, however, that Apple is offering an interim update to correct the said bug by then.
Recall that Apple exposed a workaround using Siri, and that there was also in a similar register the case of the eight characters causing Skype to crash.