Since [man]ereg_replace()[/man] is a function that is working with regular expressions and ( and ) is used in reg-exps then you will have to escape them:
<?php
// the string:
$str = "Here is some text ( nice huh? ). Let's replace it!";
// replace ( and ) with { and } using ereg_replace():
$str = ereg_replace( "\\(" , "{" , $str );
$str = ereg_replace( "\\)" , "}" , $str );
// print it:
echo $str;
?>
To make it more simple, let's use a simple replacing function, [man]str_replace()[/man]:
<?php
// the string:
$str = "Here is some text ( nice huh? ). Let's replace it!";
// replace ( and ) with { and } using str_replace():
$str = str_replace( array( "(" , ")" ) , array( "{" , "}" ) , $str );
// print it:
echo $str;
?>
Hope it helps!