X --> Y

<?php

$arr = array("H2O" => "vatten",
                    "EFP" => "Electronic field production");

echo $arr["H20"];

?>

<?php
  $arr = Array("H2O" => "Vatten", "HCl" => "Saltsyra", "Bi" => "FUCK YEAH, BISMUTH");
  echo $arr[$_GET['text']];
?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>">
  <input type="text" name="text" id="text">
  <button type="submit">Skicka</button>
</form>

$stuff= array('H2O'=>'Vatten','EFP'=>'Electronic field production');
echo $stuff['H2O'];