Are you saying that some of them work correctly and others don't? Do any of them still try to pop up a download window? What changes did you make to httpd.conf? Editing mime.types was kind of a "fall through" measure. In the first section with the AddType directives, this told apache how to handle this type of file. If it failed "IfModule" tests then it would look in mime.types to see how to handle .php files. If you put text/plain php in there, apache would serve it as plain text, which seems to be what is happening. I have been assuming that you have only been editing the original version of httpd.conf but if you used a configuration program, it may have left a lot of stuff out. If you had to add the lines I posted earlier, then there is more missing. You will have to add and load some modules. If these lines do not exist, add them to you httpd.conf right under where all the modules are loaded and added near the top of the file.
#This section goes under the "LoadModule" section
<IfDefine HAVE_PERL>
LoadModule perl_module modules/libperl.so
</IfDefine>
<IfDefine HAVE_PHP>
LoadModule php_module modules/mod_php.so
</IfDefine>
<IfDefine HAVE_PHP3>
LoadModule php3_module modules/libphp3.so
</IfDefine>
<IfDefine HAVE_PHP4>
LoadModule php4_module modules/libphp4.so
</IfDefine>
#This section goes under the "AddModule" section
<IfDefine HAVE_PERL>
AddModule mod_perl.c
</IfDefine>
<IfDefine HAVE_PHP>
AddModule mod_php.c
</IfDefine>
<IfDefine HAVE_PHP3>
AddModule mod_php3.c
</IfDefine>
<IfDefine HAVE_PHP4>
AddModule mod_php4.c
</IfDefine>
If you defined php as text/plain in mime.types you can get rid of it now. If you have all the httpd.conf stuff I have in these 2 posts, you should not have to do anything else. It should work correctly.