Useful function of php

Most useful function of php:

=> rand()   ---------    that is except value randomly not from user.
      Exam:  code
                  <?php
                  $rand= rand();
                   echo $rand;
                     ?>
        output: 21417
=> getrandmax() ------------   that is except maximum value.
<?php ob_start(); ?>
<h1> my page</h1>
<?php
    $redirect_page='http://google.com';
    $redirect=false;
    if($redirect==true){
        header('Location: '.$redirect_page);
    }
  
    ob_end_flush();
?>