I move around a lot between servers in my line of work so a quick way to see what modules are installed is very valuable. You can of course use the built in phpinfo() function but that requires a browser and web server. A quicker way for me is to use the command line interface.
Simply input on the command line:
php -m
and all the installed modules will be displayed.
Example output:
[PHP Modules] bz2 calendar ctype curl date dbase exif ftp gettext gmp hash iconv libxml mime_magic mqseries openssl pcntl pcre posix pspell Reflection session shmop SimpleXML soap sockets SPL standard sysvmsg sysvsem sysvshm tokenizer wddx xml xmlrpczlib
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