Why Does PHP Throw a Syntax Error When Initializing Class Properties with Simple Expressions?

Syntax Error During Property Initialization in PHP Classes

According to the PHP documentation, class property declarations can be initialized with constant values, meaning their evaluation doesn't depend on run-time information. However, developers may encounter issues when initializing properties with simple expressions. For example, the following code initializes an array with predefined key-value pairs:

<code class="php">public $var = array(
    1 => 4,
    2 => (4+1),
);</code>

While the first key-value pair (1 => 4) is valid, the second (2 => (4 1)) generates a syntax error. Even assigning a simple arithmetic expression to a property, such as $var = 4 1, results in a syntax error.

This behavior suggests that the limitation isn't just for specific language constructs like arrays. However, expressions like "4 1" can be evaluated at compile-time and should be considered constant values.

PHP 5.6 introduced a new feature known as constant scalar expressions, which addresses this issue. These expressions allow scalar expressions involving numeric and string literals or constants to be used in contexts where PHP previously expected static values, including property declarations.

The following code, which was previously causing a syntax error, is now valid:

<code class="php">public $var = array(
    1 => 4,
    2 => (4+1),
);</code>

This change gives developers more flexibility in initializing class properties with simple expressions that can be optimized away during compilation.

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