php_write_to_file_perf_test.php

<?PHP
  shell_exec("touch messages1.txt");
  shell_exec("touch messages2.txt");


  $time_1 = microtime(true);
  for ($i=1; $i<1000; $i++) {
    file_put_contents('messages1.txt', "This is the sample text\n", FILE_APPEND);
  }
  $time_delta_1 = microtime(true) - $time_1;
  $time_log_message_1 = "$time_delta_1 delta for file_put_contents\n";


  $time_2 = microtime(true);
  for ($i=1; $i<1000; $i++) {
    shell_exec("echo 'This is the sample text' >> messages2.txt");
  }
  $time_delta_2 = microtime(true) - $time_2;
  $time_log_message_2 = "$time_delta_2 delta for shell_exec(echo)\n\n";


  echo("$time_log_message_1<br>$time_log_message_2");


  shell_exec(": > messages1.txt");
  shell_exec(": > messages2.txt");

Fairly standard output:

0.16222381591797 delta for file_put_contents
3.1839950084686 delta for shell_exec(echo)

So … use file_put_contents.