Fatal Error - Too many open files

How can you up file open limit (Linux or Max OS):

ulimit -n 10000

Solves problem with phpunit or/and phpdbg and Warning: Uncaught ErrorException: require([..file]): failed to open stream: Too many open files in [...]

In php, before the execution, try this

exec('ulimit -S -n 2048');

This can be a limitation on the server where the code is running. Every operating system only allows for a certain number of open files/handles/sockets. This limit is usually further reduced when the server is virtualized. On a Linux server you can check the current limit with ulimit -n, if you have root access you can increase it with the same command. I assume there is a method for Windows server as well. Otherwise there is not much you can do about it (except ask your hoster or administrator to increase it).

More configurable limits:

Change in /etc/security/limits.conf

soft nofile 1024
hard nofile 65535

Increase ulimit by ulimit -n 65535 or echo 65535 > /proc/sys/fs/file-max or in /etc/sysctl.conf:

fs.file-max=65535