Warning: Cannot modify header information

Getting this error when trying to update pages.

Warning: strtolower() expects parameter 1 to be string, array given in /home/mzptv/public_html/wp-includes/formatting.php on line 772

Warning: Cannot modify header information – headers already sent by (output started at /home/mzptv/public_html/wp-includes/formatting.php:772) in /home/mzptv/public_html/wp-includes/pluggable.php on line 934

I’ve done a search

and read the other support topics about “Cannot modify header information”, and I’ve tried removing spaces from pluggable.php, formatting.php, header.php and wp-config.php, but the error remains.

Line 772 of formatting.php is —

$key = strtolower( $key );

The line in context is —

function sanitize_key( $key ) {
	$raw_key = $key;
	$key = strtolower( $key );
	$key = preg_replace( '/[^a-z0-9_\-]/', '', $key );
	return apply_filters( 'sanitize_key', $key, $raw_key );
}

Let me know if there’s any other code I can drop on paste bin to help fix this. I’m a PHP newb.