<?php 

function IsIPInNet( $ip, $net ) { 
if( preg_match( '/^([^\/]+)\/([^\/]+)$/', $net, $ms ) ) { 
$mask = 0xFFFFFFFF << ( 32 - $ms[2] ); 
return ( ip2long( $ip ) & $mask ) == ( ip2long( $ms[1] ) & $mask ); 
} 
return false; 
} 

echo IsIPInNet( '1.2.3.4', '1.2.3.0/24' )? "yes" : "no"; 
echo IsIPInNet( '2.2.3.4', '1.2.3.0/24' )? "yes" : "no"; 

?>