Where is this bit of PHP supposed to go?
If it’s supposed to go into the <head>
of a document, use an action hook in your child themes functions file to hook it to wp_head()
On the other hand, if it’s supposed to go anywhere in a post, then you have two options:
1) Use the HTML editor to insert it in each and every post separately (which is a pain if you don’t like to use the html editor, plus you need to do on each and every post).
2) Use a Graphene action hook widget area along with a text widget. Text widgets won’t let you use PHP code, however, so you’ll need to use a PHP widget to allow you to enter PHP in a text widget. See this tutorial on how to do that.