Watch out for...

✔ default charset is UTF-8 instead of ISO-8859-1

<?php
echo htmlspecialchars("abc".chr(0xE0)."def"); // 5.3 "abc�def"
echo htmlspecialchars("abc".chr(0xE0)."def"NULL'UTF-8'); // 5.3 ""
echo htmlspecialchars("abc".chr(0xE0)."def"); // 5.4 ""
echo htmlspecialchars("abc".chr(0xE0)."def"ENT_IGNORE); // 5.4 "abcdef"

✔ array to string conversion notice

<?php
echo [1,2,3];
// Notice: Array to string conversion in test.php code on line 1

array_diff([1,2,3], [1,2, [3] ]);
// Notice: Array to string conversion in test.php on line 2

✔ register_globals completely removed


✔ magic quotes completely removed


✔ removed variable break/continue

<?php
$var 
'label';
while(
1) {
  break 
$var;
  continue 
$var;
}
label:

✔ max_input_vars (default 1000)


✔ "callable", "insteadof" and "trait" are now reserved words


✔ extending an abstract constructor must match the signature

<?php
  
abstract class Base {
    abstract public function 
__construct();
  }
  class 
Foo extends Base {
    public function 
__construct($bar) {}
// FATAL  Declaration of Foo::__construct() must be compatible 
//        with Base::__construct()
  
}

✔ stream_select() preserves keys of array arg


✔ XSLT writes are disabled by default


✔ mcrypt_generic_end() removed in favour of mcrypt_generic_deinit()


✔ mysql_list_dbs() removed


✔ PDO needs at least mysql client lib 4.1 now


✔ Datetime ignores TZ env var and falls back to UTC if none set


✔ Tiger hash output fixed - See bug 61307