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<?php

http://www.marketingextremist.com/how-to-fix-htmlspecialchars-error-after-updating-to-wordpress-3-6-579/

http://wpgyan.com/resolve-warning-htmlspecialchars-charset-utf-7-not-supported-assuming-utf-8/

1)  You Can see the changed charset in WP admin Dashboard in   Settings => Reading.
- Change to UTF-8 if it was changed , else proceed with (2)

2) 
Log in to your FTP or Cpanel and go to the /wp-includes folder which could be found within your root directory.

Open the file formatting.php

Find the line 2751 and turn off the warnings by placing @” before the function:

before: $safe_text = htmlspecialchars( $text, ENT_QUOTES, get_option(‘blog_charset’) );

after: $safe_text = @htmlspecialchars( $text, ENT_QUOTES, get_option(‘blog_charset’) );

LINE: 3973
function esc_textarea( $text ) {
  // supress the warning
	// $safe_text = htmlspecialchars( $text, ENT_QUOTES, get_option( 'blog_charset' ) ); change this
	$safe_text = @htmlspecialchars( $text, ENT_QUOTES, get_option(‘blog_charset’) );
	/**
	 * Filters a string cleaned and escaped for output in a textarea element.
	 *
	 * @since 3.1.0
	 *
	 * @param string $safe_text The text after it has been escaped.
 	 * @param string $text      The text prior to being escaped.
	 */
	return apply_filters( 'esc_textarea', $safe_text, $text );
}