Description:
------------
str_replace makes correkt changes. everay time i make an echo with the var it is coorect,

But sometimes if i use the result value as a params to another function the value is null.

Reproduce code:
---------------
 $neu= "ALTER TABLE `cms_content_props` ADD INDEX ( `prop_name` )  ";
 $sql=str_replace('`cms_',"`".$config['db_prefix'],$neu);
 echo $sql; // it shows always the correct value
 $db->Execute($sql);  // but in function $sql has a value uf NULL.

// this occurs after 10 ..14 times working with the same sequence but different values of $neu

if i use this

  $neu= "ALTER TABLE `cms_content_props` ADD INDEX ( `prop_name` )  ";
  $db->Execute(str_replace('`cms_',"`".$config['db_prefix'],$neu));


no error