Before you can overwrite the php.ini values, you need to have the following two lines present at the top of your .htaccess file to make the changes work.
Following is the way to set values in php.ini using .htaccess. For example if you want to enable PHP error log and turn on the error display using .htaccess you will need to write the following in your .htaccess file.
There are two kind of directives which can be set in .htaccess. php_value for the string type of values and php_flag for the boolean values. In the above example we used later to set the errors on which is a boolean value. An example to set php_value would be to change the default memory limit and default post size.
You can change the values of various options listed in php.ini via .htaccess. Hope this helps.