No, via .htaccess, you can't change php variables. But, you can changes php variables with the help of a .user.ini file. Create that file in the rootfolder of your domain and enter the desired settings into the file.
Please make sure not to enter too large values (exaggerated values won't be accepted by the host). You can check whether your individual settings are accepted by creating a new webpage (e.g. yourdomain.com/settings) and entering the follwing code in the new webpage: phpinfo().
Here are few examples of the way you need to enter the php variables with individual values in the file .user.ini:
This sets the maximum amount of memory in bytes that a script is allowed to allocate. This helps prevent poorly written scripts for eating up all available memory. The Servage max. value is 96M.
memory_limit = 96M
Max Upload Size
The maximum size of an uploaded file, this may required if you want to upload larger images or attachments.
upload_max_filesize = 40M
Max Execution Time
This sets the maximum time in seconds a script is allowed to run before it is terminated by the parser.
Max Input Time
This sets the maximum time in seconds a script is allowed to parse input data, like POST and GET.
Max Post Size
Sets max size of post data allowed. This setting also affects file upload. To upload large files, this value must be larger than upload_max_filesize.