I can't run PHP scripts in wordpress pages

This is work of WP auto paragraph function.
Learn more about wpautop and wptexturize functions.

To disable the autop completely in content paste this code to functions.php
remove_filter( 'the_content', 'wpautop' );

for more flexible settings check
http://wordpress.org/extend/plugins/raw-html/
http://wordpress.org/extend/plugins/ps-disable-auto-formatting/
http://www.wpbeginner.com/wp-tutorials/how-to-disable-automatic-formatting-in-wordpress-posts/

there is another one good plugin to run PHP via [shortcode]
http://wordpress.org/extend/plugins/shortcode-exec-php/