replace_content

<?php
$content ='
	<h1 id="my_id" style="font-weight:bold" class="my_class">This is h1</h1>
	<div class="my_desc">This is example</div>
	<h1 class="h2_class">This is h1 no class</h1>
	<h2 style="color:red;">This is h2</h2>
';

function replace_content($content) {
	$content = preg_replace( '/<h(\d{1,6})(.*?)>(.*?)<\/h(\d{1,6}).*?>/', '<h$1 $2><span>$3</span></h$4>',  $content );
	$content = custom_strip_attr($content, array('h1', 'h2', 'h3', 'h4', 'h5', 'h6'), array('style'));
	return $content;
}
function custom_strip_attr( $text, $elements, $attributes, $two_passes = true ) {
	$elements_pattern = implode( '|', (array) $elements );

	/** Build patterns */
	$patterns = array();
	foreach ( (array) $attributes as $attribute ) {
		/** Opening tags */
		$patterns[] = sprintf( '~(<(?:%s)[^>]*)\s+%s=[\\\'"][^\\\'"]+[\\\'"]([^>]*[^>]*>)~', $elements_pattern, $attribute );

		/** Self closing tags */
		$patterns[] = sprintf( '~(<(?:%s)[^>]*)\s+%s=[\\\'"][^\\\'"]+[\\\'"]([^>]*[^/]+/>)~', $elements_pattern, $attribute );
	}

	/** First pass */
	$text = preg_replace( $patterns, '$1$2', $text );

	if ( $two_passes ) /** Second pass */
		$text = preg_replace( $patterns, '$1$2', $text );
	return $text;

}
echo replace_content($content);