Basically you would want to install php5-core and any extensions you want or need beyond that (e.g. php5-mysql). The PHP interpreter (php) can be used stand alone for testing simple scripts but for most interesting stuff related to the web: you will need to do as Oko has said - and set it up for use with Apache via module or CGI. Using PHP with Apache on OpenBSD is trivial if the chroot is disabled, if apache/httpd is run under chroot, well then it gets a little more interesting
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.