PHP is immune

 [/Users/chris/Code/Ruby/gist-550144 (master)] ruby-1.9.2-p0
$ time php php_string.php > /dev/null

real	0m0.237s
user	0m0.218s
sys	0m0.017s
 [/Users/chris/Code/Ruby/gist-550144 (master)] ruby-1.9.2-p0
$ time php php_array.php > /dev/null

real	0m0.249s
user	0m0.221s
sys	0m0.023s

php_array.php

<?php
$names = array('Fred', 'Bob', 'Harry', 'Jim', 'Tony', 'Alf', 'Richard', 'John', 'David', 'Ralf', 'Peter');
$bigArray = array();

for ($i = 1; $i <= 50000; $i++) {
    $bigArray[] = $names[rand(0, count($names) - 1)];
}

echo implode("\n", $bigArray);

php_string.php

<?php
$names = array('Fred', 'Bob', 'Harry', 'Jim', 'Tony', 'Alf', 'Richard', 'John', 'David', 'Ralf', 'Peter');
$bigString = "";

for ($i = 1; $i <= 50000; $i++) {
    $bigString .= $names[rand(0, count($names) - 1)] . "\n";
}

echo $bigString;

Ruby arrays are quick

 [/Users/chris/Code/Ruby/string_vs_array] ruby-1.9.2-p0
$ time ruby string.rb > /dev/null

real	0m5.545s
user	0m2.553s
sys	0m2.921s
 [/Users/chris/Code/Ruby/string_vs_array] ruby-1.9.2-p0
$ time ruby array.rb > /dev/null

real	0m0.047s
user	0m0.035s
sys	0m0.009s

ruby_array.rb

names = %w{Fred Bob Harry Jim Tony Alf Richard John David Ralf Peter}
big_array = []

1.upto(50000) do
  big_array << names[rand(names.size)]
end

puts big_array.join("\n")

ruby_string.rb

names = %w{Fred Bob Harry Jim Tony Alf Richard John David Ralf Peter}
big_string = ""

1.upto(50000) do
  big_string += "#{names[rand(names.size)]}\n"
end

puts big_string

This issue isn't unique to ruby, lots of languages seem to behave the same and my crude understanding is that it's something to do with memory allocation and the heap. We used to have the same problem with classic asp and VB and a quick search suggests it's an issue in Python as well http://www.skymind.com/~ocrow/python_string/

PHP doesn't suffer from this http://stackoverflow.com/questions/124067/php-string-concatenation-performance

PHP is certainly faster overall in my very synthetic benchmark. Weirdly it seems (ever so slightly) slower using an array, and neither string nor array in PHP can match the raw speed of the Ruby array...