The following code should allow for other Smarty delimiter tags to be used with your substitution:
$source_content = preg_replace_callback($search,
function($m) {
return $this->_quote_replace($this->left_delimiter)."php ".str_repeat("\n", substr_count($m[0], "\n")).$this->_quote_replace($this->right_delimiter);
},
$source_content);