php-doc-en/language/oop5/properties.xml

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 <sect1 xml:id="language.oop5.properties" xmlns="http://docbook.org/ns/docbook">
  <title>Properties</title>

  <para>
   Class member variables are called <emphasis>properties</emphasis>.
   They may be referred to using other terms such as <emphasis>fields</emphasis>,
   but for the purposes of this reference <emphasis>properties</emphasis>
   will be used. They are defined by using one of the keywords
   <literal>public</literal>, <literal>protected</literal>, or
   <literal>private</literal>, optionally, as of PHP 7.4,
   followed by a type declaration, followed by a normal variable declaration.
   This declaration may include an initialization, but this initialization
   must be a <link linkend="language.constants">constant</link> value.
  </para>
  <para>
   See <xref linkend="language.oop5.visibility" /> for more
   information on the meanings
   of <literal>public</literal>, <literal>protected</literal>,
   and <literal>private</literal>.
  </para>
  <note>
   <para>
    An alternative and not recommended way of declaring class properties, as it is to maintain backward
    compatibility with PHP 4, is by using
    the <literal>var</literal> keyword.
    It will treat the property identically as it would have been
    declared as <literal>public</literal>.
   </para>
  </note>
  <para>
   Within class methods non-static properties may be accessed by using 
   <literal>-&gt;</literal> (Object Operator): <varname>$this-&gt;property</varname> 
   (where <literal>property</literal> is the name of the property). 
   Static properties are accessed by using the <literal>::</literal> (Double Colon):
   <varname>self::$property</varname>. See <link linkend="language.oop5.static">Static Keyword</link> 
   for more information on the difference between static and non-static properties.
  </para>
  <para>
   The pseudo-variable <varname>$this</varname> is available inside
   any class method when that method is called from within an object
   context. <varname>$this</varname> is the value of the calling
   object (usually the object to which the method belongs, but
   possibly another object, if the method is called
   <link linkend="language.oop5.static">statically</link> from the context
   of a secondary object).
  </para>

  <para>
   <example>
    <title>Property declarations</title>
    <programlisting role="php">
<![CDATA[
<?php
class SimpleClass
{
   public $var1 = 'hello ' . 'world';
   public $var2 = <<<EOD
hello world
EOD;
   public $var3 = 1+2;
   // invalid property declarations:
   public $var4 = self::myStaticMethod();
   public $var5 = $myVar;

   // valid property declarations:
   public $var6 = myConstant;
   public $var7 = [true, false];

   public $var8 = <<<'EOD'
hello world
EOD;
}
?>
]]>
    </programlisting>   
   </example>
  </para>

  <note>
   <para>
    There are some nice functions to handle classes and objects. You
    might want to take a look at
    the <link linkend="ref.classobj">Class/Object Functions</link>.
   </para>
  </note>

  <sect2 xml:id="language.oop5.properties.typed-properties">
   <title>Type declarations</title>
   <para>
    As of PHP 7.4.0, property definitions can include a
    <xref linkend="language.types.declarations" />,
    with the exception of <type>callable</type>.
    <example>
     <title>Example of typed properties</title>
     <programlisting role="php">
<![CDATA[
<?php

class User
{
    public int $id;
    public ?string $name;

    public function __construct(int $id, ?string $name)
    {
        $this->id = $id;
        $this->name = $name;
    }
}

$user = new User(1234, null);

var_dump($user->id);
var_dump($user->name);

?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
int(1234)
NULL
]]>
     </screen>
    </example>
   </para>

   <para>
    Typed properties must be initialized before accessing, otherwise an
    <classname>Error</classname> is thrown.
    <example>
     <title>Accessing properties</title>
     <programlisting role="php">
<![CDATA[
<?php

class Shape
{
    public int $numberOfSides;
    public string $name;

    public function setNumberOfSides(int $numberOfSides): void
    {
        $this->numberOfSides = $numberOfSides;
    }

    public function setName(string $name): void
    {
        $this->name = $name;
    }

    public function getNumberOfSides(): int
    {
        return $this->numberOfSides;
    }

    public function getName(): string
    {
        return $this->name;
    }
}

$triangle = new Shape();
$triangle->setName("triangle");
$triangle->setNumberofSides(3);
var_dump($triangle->getName());
var_dump($triangle->getNumberOfSides());

$circle = new Shape();
$circle->setName("circle");
var_dump($circle->getName());
var_dump($circle->getNumberOfSides());
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
string(8) "triangle"
int(3)
string(6) "circle"

Fatal error: Uncaught Error: Typed property Shape::$numberOfSides must not be accessed before initialization
]]>
     </screen>
    </example>
   </para>
  </sect2>

 </sect1>
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