Test for memory usage and bail out at a certain percent

memory_limit_script.php

function test_memory_limit() {
  // Using drush but any arbitrary number for memory limit will do.
  $mem_limit = drush_memory_limit();
  
  $bail_percentage = 90;
  
  $big_array = [];
  $i = 9999;
  while (TRUE) {
    $i++;

    // Just some string to make the array exponentially larger on each loop.
    $big_array[] = str_repeat(uniqid() . uniqid(), $i);

    // Memory check - 90% of total.
    if (memory_get_usage(TRUE) >= ($bail_percentage/100*$mem_limit)) {
      // Log errors or notify whoever that the process couldn't complete.
      var_dump(memory_get_usage(TRUE));
      echo 'This PHP script has been killed for using too much memory. An email has been sent.';
      die();
    }
  }

}