warning: preg_match() expects parameter 2 to be string, array given in (site)/bootstrap.inc on line 737. appears on all pages

This normally stings me when I double up on a form element. How it got me: I was working on a module that displayed user profiles, so I started off with the profile.module and got hacking (this was not only my first Drupal code ever, also my first PHP).
To cut a long story short, when accessing pages that had common elements between my module and profile.module, there were two elements that had the same name and so PHP had avoided throwing an error by making the two strings into an array.

To do some troubleshooting on this, start disabling modules till it starts working again. The direct path (to side step any errors like you've seen) is /admin/build/modules

Here's a patch that modifies drupal_attributes() to support attribute values that are arrays.

I just got hit with what is essentially the same error with the project bundle:

warning: preg_match() expects parameter 2 to be string, array given in htdocs/includes/bootstrap.inc on line 777.

From looking at many other reports for this and similar errors, and looking at the code, it seems to me that the culprit is the check_plain function itself.

function check_plain($text) {
  return drupal_validate_utf8($text) ? htmlspecialchars($text, ENT_QUOTES) : '';
}

That function is called for many different reasons. This error is going to be produced whenever it is called to check a display field that potentially has an array value such as is provided by a pulldown select field.

I believe the correct fix for this, which will fix many different contrib modules at thesame time, is to have check_plain detect an array and then call drupal_validate_utf8 on each member of the array, replaceing as needed. Here is the logic I am proposing:

function check_plain($text) {
  if ( $text a string )
    // keep original functionality
    return drupal_validate_utf8($text) ? htmlspecialchars($text, ENT_QUOTES) : '';
  else if ( $text is reall an array ) {
    foreach $member in $text {
      $member = drupal_validate_utf8($text) ? htmlspecialchars($text, ENT_QUOTES) : '';
      replace original value in $text array with $member;
    }
  }
}

I am not a PHP person, I work mainly in perl and c, or I would do this myself.

Again, a search for "warning: preg_match() expects parameter 2 to be string" indicates that this is a very common problem and the intermitent nature is driving people nuts.

boftx: I think it's perfectly acceptable for a string function to only handle strings, it's the responsibility of other code to ensure that only strings are passed to it - most languages work this way, a does PHP itself.

I agree with your statement in principle, and in my own shop would enforce that ruthlessly.

But given how widespread this is, and the apparent ability for code to call check_plain with an array unbeknownst to the programmer, I think some defensive action is called for. Most of the reports I have read stem from using a view or CCK fields. This implies that an end-user could conceivably create an error of this type simply by changing an existing field.

Because of this I submit for consideration that this function, check_plain, needs to be re-implemented to handle arrays as well as strings. A review of some of the modules having this problem will probably indicate what the expected return needs to be when called with an array. It might well be that simply modifying the values in-place will be sufficient.

If nothing else, an error message with a reasonable back trace should be produced.

Thank you for responding.

hi,
same here,

what do you think about that solution?
http://drupal.org/node/525036

i dont like but i need to solve it.
regards

following, after many hours trying to get behind this problem after upgrading to Drupal6.15 and many recommended module upgrades:

How diagnose: warning: preg_match() expects parameter 2 to be string, array given in .../includes/bootstrap.inc 777