export LD_LIBRARY_PATH=/usr/local/lib

[root@xxx ~]#protoc --php_out=/root/test_protobuf/ test.proto
protoc: error while loading shared libraries: libprotobuf.so.15: cannot open shared object file: No such file or directory
[root@xxx ~]#export LD_LIBRARY_PATH=/usr/local/lib
[root@xxx ~]#protoc --php_out=/root/test_protobuf/ test.proto

[root@xxx ~]#cd /root/test_protobuf/
[root@xxx test_protobuf]#ll
total 16
-rw-r--r-- 1 root root 1193 Mar  6 14:47 AddressBook.php
drwxr-xr-x 2 root root 4096 Mar  6 14:47 GPBMetadata
-rw-r--r-- 1 root root 3694 Mar  6 14:47 Person.php
-rw-r--r-- 1 root root 1866 Mar  6 14:47 PhoneNumber.php