Use implode instead of join
Don't use $key, rename $value to $char

- Let's avoid the creation of the temporary array $values
- Don't assign the array index ($key) since we don't use it
- Rename $value to $char: it's more clear that it's a char of $format
Coding style: respect max line length of 80
Rename $temp to $converted

Let's make clear what that variable contains
Normalize control structures

Let's avoid mixing 'continue' and 'if/else': let's always use 'if/else'
Rename $lastescaped to $inEscapedString

Let's make clear the meaning of this variable
Add some comments
Add tests for non simple convertPhpToIsoFormat