你可以试试这个吗，

    $string ="Hello <%First Name%> <%Last Name%> welcome";
    preg_match_all('~<%(.*?)%>~s',$string,$datas);
    $Array = array('0' => array ('First Name' => 'John', 'Last Name' => 'Smith' ));
    $Html =$string;
    foreach($datas[1] as $value){           
        $Html =str_replace($value, $Array[0][$value], $Html);
    }
    echo str_replace(array("<%","%>"),'',$Html);