Given a directory DIR and a glob of files (e.g. *.php):

$ svn propset svn:ignore \*.php DIR

You can also remove a property:

$ svn propdel svn:ignore DIR

The svn propset format only allows you to set one pattern at a time. To add multiple patterns, run:

$ svn propedit svn:ignore DIR

which opens a file in an editor where you can add ignore patterns, one per line.