preg replace placeholders

I'm trying to append a 开发者_StackOverflow社区number to another number

$n = 123;
$str = "some string with tt=789_ and more";
echo preg_replace("/tt=[0-9]+/", "$0$n", $str);

I expect this to print "some string with tt=789_123 and more" for some reason i'm getting "some string with 23_ and more".

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In your example the $0$n is transformed to $0123 which can confuse preg_replace (see the section about replacement).

So the correct way is to do the following:

$n = 123;
$str = "some string with tt=789_ and more";
echo preg_replace("/tt=[0-9_]+/", "\${0}$n", $str);

I've also added _ to your character class otherwise it returns tt=789123_.