Description:
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From manual page: http://www.php.net/function.str-replace#refsect1-function.str-replace-parameters
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> If search and replace are arrays, then str_replace() takes a value from each array and uses them to search and replace on subject. If replace has fewer values than search, then an empty string is used for the rest of replacement values. If search is an array and replace is a string, then this replacement string is used for every value of search. The converse would not make sense, though.

That last sentence is what bothers me, as I have a use case where it DOES make sense. In the patch below (line 646), I assumed that this is an accepted behavior:

https://github.com/EllisLab/CodeIgniter/commit/10cbdf091b3cdbc72847dad28a1dce03a92119b6

To shortly explain what the patched method does - it's basically a pseudo prepared statement compiler. It takes an SQL query and an array of values and then replaces each occurence of '?' (default value for $this->bind_marker) with the corresponding (by position) element from the input array.

Test script:
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<?php
$query = 'UPDATE `table_name` SET `field1_name` = :? WHERE `field2_name` = :?';
$bind_marker = '?';
$bind_values = array("'foo'", "'bar'");
echo str_replace($bind_marker, $bind_values, $sql);

Expected result:
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UPDATE `table_name` SET `field1_name` = 'foo' WHERE `field2_name` = 'bar'

Actual result:
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PHP Notice:  Array to string conversion in php shell code on line 5
UPDATE `table_name` SET `field1_name` = Array WHERE `field2_name` = Array