Help with a simple str_replace() problem

[rob76]

I am trying to remove all letters from a post/zip code leaving only numbers. $postcode is the original string from user form input and I want to return $num as just the numeric characters in the string. The following looks obvious to me but just doesn't work for some reason.
<?
$letters = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
foreach($letters as $var){
$num = str_replace($var,'',$postcode);
}
echo $num;
?>

Can anyone help?

Thanks
Rob

[planetsim]

Try using eregi_replace only because its case insensitive

Secondly try this code

<?php

$letters = array('a',
'b','c','d'
,'e','f',
'g','h','i',
'j','k','l',
'm','n','o',
'p','q','r',
's','t','u',
'v','w','x',
'y','z');

foreach($letters as $var){
  $postcode = str_replace($var,'',$postcode);
}
echo $postcode;
?> 

That works

[laserlight]

You could try this solution, which transfers each numeric character from $zipcode to a temporary string, $new_zipcode, then re-assigns the string.

$zipcode = 'some123zip';
$len = strlen($zipcode);
$new_zipcode = '';
$n = 0;
for ($i = 0; $i < $len; $i++) {
	if (ctype_digit($zipcode{$i})) {
		$new_zipcode{$n} = $zipcode{$i};
		$n++;
	}
}
$zipcode = $new_zipcode;

Conversely, you could use a regular expression, which replaces each character that is not 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 with a blank.

$zipcode = preg_replace('/[^0-9]/', '', $zipcode);

That would probably be what you intended with str_replace().

[rob76]

Thanks both for your help. I've now got it working
Rob