php_value and php_flag directives only work in an .htaccess file depending on your Server type and if your Host has specifically allowed this. I do not believe using ini_set in your wp-config.php file will work. Create a custom php.ini file and add these directives below in your custom php.ini file. If you are not sure how to create a custom php.ini file then contact your host and they will create a custom php.ini file for your website and add the 64M value.
http://stackoverflow.com/questions/2184513/php-change-the-maximum-upload-file-size
You need to set the value of upload_max_filesize and post_max_size in your php.ini :
; Maximum allowed size for uploaded files.
upload_max_filesize = 40M; Must be greater than or equal to upload_max_filesize
post_max_size = 40M
If you can’t change your php.ini, you’re out of luck. You cannot change these values at run-time; uploads of file larger than the value specified in php.ini will have failed by the time execution reaches your call to ini_set.