Why Does PHP Allow Array Interpolation Without Quotes or Brackets?

String Interpolation of Associative Arrays in PHP

The interpolation of PHP string-indexed array elements exhibits various behaviors that may surprise users. When interpolating array elements within double-quoted strings, the following observations can be made:

Using the array element's index within curly braces ($ha['key1']) or brackets ($ha[key1]) produces the expected result and results in a warning (use of undefined constant) being emitted.

However, interpolating the array element without enclosing its index in quotes or brackets ($ha[key1]) also results in the correct output, despite being syntactically unexpected.

This raises the question of why the last statement in the code snippet provided below is considered valid PHP syntax:

<code class="php">print "He said $ha[key1]";</code>

Explanation

This behavior is documented in PHP's manual and is considered a trusted feature. While the syntax may seem unconventional, it is a valid way to interpolate array elements within double-quoted strings.

Inconsistency

The documentation acknowledges that PHP's syntax has evolved over time, leading to inconsistencies. In this case, the ability to interpolate array elements without enclosing their indices in quotes or brackets is a reflection of PHP's historical development and is not a recommended practice for new code.

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