对@PeterFox 的回答稍作修改,包括在bcmath 不可用时支持bcmod(),
<?php
function isValidIBAN ($iban) {
$iban = strtolower($iban);
$Countries = array(
'al'=>28,'ad'=>24,'at'=>20,'az'=>28,'bh'=>22,'be'=>16,'ba'=>20,'br'=>29,'bg'=>22,'cr'=>21,'hr'=>21,'cy'=>28,'cz'=>24,
'dk'=>18,'do'=>28,'ee'=>20,'fo'=>18,'fi'=>18,'fr'=>27,'ge'=>22,'de'=>22,'gi'=>23,'gr'=>27,'gl'=>18,'gt'=>28,'hu'=>28,
'is'=>26,'ie'=>22,'il'=>23,'it'=>27,'jo'=>30,'kz'=>20,'kw'=>30,'lv'=>21,'lb'=>28,'li'=>21,'lt'=>20,'lu'=>20,'mk'=>19,
'mt'=>31,'mr'=>27,'mu'=>30,'mc'=>27,'md'=>24,'me'=>22,'nl'=>18,'no'=>15,'pk'=>24,'ps'=>29,'pl'=>28,'pt'=>25,'qa'=>29,
'ro'=>24,'sm'=>27,'sa'=>24,'rs'=>22,'sk'=>24,'si'=>19,'es'=>24,'se'=>24,'ch'=>21,'tn'=>24,'tr'=>26,'ae'=>23,'gb'=>22,'vg'=>24
);
$Chars = array(
'a'=>10,'b'=>11,'c'=>12,'d'=>13,'e'=>14,'f'=>15,'g'=>16,'h'=>17,'i'=>18,'j'=>19,'k'=>20,'l'=>21,'m'=>22,
'n'=>23,'o'=>24,'p'=>25,'q'=>26,'r'=>27,'s'=>28,'t'=>29,'u'=>30,'v'=>31,'w'=>32,'x'=>33,'y'=>34,'z'=>35
);
if (strlen($iban) != $Countries[ substr($iban,0,2) ]) { return false; }
$MovedChar = substr($iban, 4) . substr($iban,0,4);
$MovedCharArray = str_split($MovedChar);
$NewString = "";
foreach ($MovedCharArray as $k => $v) {
if ( !is_numeric($MovedCharArray[$k]) ) {
$MovedCharArray[$k] = $Chars[$MovedCharArray[$k]];
}
$NewString .= $MovedCharArray[$k];
}
if (function_exists("bcmod")) { return bcmod($NewString, '97') == 1; }
// http://au2.php.net/manual/en/function.bcmod.php#38474
$x = $NewString; $y = "97";
$take = 5; $mod = "";
do {
$a = (int)$mod . substr($x, 0, $take);
$x = substr($x, $take);
$mod = $a % $y;
}
while (strlen($x));
return (int)$mod == 1;
}