Search and replace multiple values with multiple/different values in PHP5?

$string = 'blah blarh bleh bleh blarh';
$trans = array("blah" => "blerh", "bleh" => "blerh");
$result = strtr($string,$trans);

You can check the manual for detailed explanation.

You are looking for str_replace().

$string = 'blah blarh bleh bleh blarh';
$result = str_replace(
  array('blah', 'blarh'), 
  array('bleh', 'blerh'), 
  $string
);

// Additional tip:

And if you are stuck with an associative array like in your example, you can split it up like that:

$searchReplaceArray = array(
  'blah' => 'bleh', 
  'blarh' => 'blerh'
);
$result = str_replace(
  array_keys($searchReplaceArray), 
  array_values($searchReplaceArray), 
  $string
);