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function convert_smart_quotes( $string ) { 
    $search = array(chr(145), 
                    chr(146), 
                    chr(147), 
                    chr(148), 
                    chr(151)); 
 
    $replace = array("'", 
                     "'", 
                     '"', 
                     '"', 
                     '-'); 
 
    return str_replace( $search, $replace, $string ); 
}

OU

str_replace( array( chr(145), chr(146), chr(147), chr(148), chr(151)), array("'", "'", '"', '"', '-') , $string );