- str_replace: documentation ( source)
<?php
$my_string = "T E S T R E N T A L A K T I E B O L A G";
$string_a = str_replace(' ','+',$my_string);
$string_b = str_replace(' ','',$string_a );
$final_string = str_replace('+',' ',$string_b);
echo $final_string;