The only errors that may occur are due to codes used in events, libraries, and methods that are not compatible with PHP 7.0. These errors must be corrected manually.
1 - Short open tags
The short_open_tag directive tells PHP whether the abbreviated form (<? ?>) of the PHP opening tag is allowed.
Regardless of the php version, you need to check in php.ini if short_open_tags is enabled.
Solution 1:
Change the opening tag of php to (<?php ?>)
Solution 2:
Change the short_open_tags directive in php.ini:
short_open_tags = On2 - New objects cannot be assigned by reference
<?php
class C {}
$c =& new C;
?>
The example above will print in PHP 5:
Deprecated: Assigning the return value of new by reference is deprecated in /tmp/test.php on line 3
The above example will print in PHP 7:
Parse error: syntax error, unexpected 'new' (T_NEW) in /tmp/test.php on line 3
Solution:
Remove reference (&) from assignment:
<?php
class C {}
$c = new C;
?>
3 - PHP 4 Style Constructors
PHP 4 style constructors (methods that have the same name as the class where they are defined) are deprecated, and will be removed in the future. PHP 7 will issue E_DEPRECATED if a PHP 4 constructor is the only constructor defined in the class. Classes that implement the __construct () method are not affected.
<?php
class foo {
function foo() {
echo 'Eu sou um construtor';
}
}
?>
The above example will print:
Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; foo has a deprecated constructor in example.php on line 3
4 - Static calls to non-static methods
Static calls to methods that have not been declared as static are deprecated, and can be removed in the future.
<?php
class foo {
function bar() {
echo 'Eu não sou estático!';
}
}
foo::bar();
?>
The above example will print:
Deprecated: Non-static method foo::bar() should not be called statically in - on line 8
I am not static!
For more information:
- PHP 7.0 new features
- PHP 7.0 deprecated features
Source: http://php.net