An invaluable function is memory_get_usage. This function returns the amount of memory your script is being used.

As an example check how much memory giving a value to a variable will use:

  //start memory
  $mem_start = memory_get_usage();
 
  $s = "This is a test variable.";  
 
  //end memory
  $mem_end = memory_get_usage();
 
  //total used
  $used = $mem_end - $mem_used;
  echo $used;

You can use this to check if there are any potential memory problems with your script.