The "/dev/urandom" is a place on the server, that allows anybody (that includes PHP scripts) to get cryptographically secure true random numbers.
When "open_basedir" "php.ini" setting is enabled on the Web Server in general or inside Virtual Host configuration in particular it will ultimately block PHP scripts from accessing "/dev/urandom" file and make it less secure because of it.
Solution
At the "System Requirements" installation wizard step for cases, when it's not a Windows and "open_basedir" setting in "php.ini" is enabled ensure, that "/dev/urandom" is present in there as well.
Quote: 0.5h