You can use lsof to find open logfiles on your system. lsof just gives you a list of all open files.
Use grep for "log" ... use grep again for "php" (if the filename contains the strings "log" and "php" like in "php_error_log" and you are the root user you will find the files without knowing the configuration).
lsof | grep log
... snip
gmain 12148 12274 user 13r REG 252,1 32768 661814 /home/user/.local/share/gvfs-metadata/home-11ab0393.log
gmain 12148 12274 user 21r REG 252,1 32768 662622 /home/user/.local/share/gvfs-metadata/root-56222fe2.log
gvfs-udis 12246 user mem REG 252,1 55384 790567 /lib/x86_64-linux-gnu/libsystemd-login.so.0.7.1
==> apache 12333 user mem REG 252,1 55384 790367 /var/log/http/php_error_log**
... snip
lsof | grep log | grep php
**apache 12333 user mem REG 252,1 55384 790367 /var/log/http/php_error_log**
... snip
Also see this article on finding open logfiles: Find open logfiles on a Linux system
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