fatal error bei den Beiträgen

Beim Anschauen bestehender und neuer Beiträge erhalte ich folgende Fehlermeldung:

Fatal error: Uncaught Error: Call to undefined function mysql_real_escape_string() in /homepages/5/d453102612/htdocs/wp-content/themes/twentyeleven-child/header.php:6 Stack trace: #0 /homepages/5/d453102612/htdocs/wp-content/themes/twentyeleven-child/header.php(151): get_page_id() #1 /homepages/5/d453102612/htdocs/wp-includes/template.php(688): require_once(‚/homepages/5/d4…‘) #2 /homepages/5/d453102612/htdocs/wp-includes/template.php(647): load_template(‚/homepages/5/d4…‘, true) #3 /homepages/5/d453102612/htdocs/wp-includes/general-template.php(41): locate_template(Array, true) #4 /homepages/5/d453102612/htdocs/wp-content/themes/twentyeleven-child/single.php(10): get_header() #5 /homepages/5/d453102612/htdocs/wp-includes/template-loader.php(74): include(‚/homepages/5/d4…‘) #6 /homepages/5/d453102612/htdocs/wp-blog-header.php(19): require_once(‚/homepages/5/d4…‘) #7 /homepages/5/d453102612/htdocs/index.php(17): require(‚/homepages/5/d4…‘) #8 {main} thrown in /homepages/5/d453102612/htdocs/wp-content/themes/twentyeleven-child/header.php on line 6

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