Why does PHP\'s string interpolation work with associative arrays even without curly braces?

Interpolation of Associative Arrays in PHP: Uncovering Unexpected Behavior

PHP's string interpolation offers a convenient way to embed variables into strings. However, when working with associative arrays, interpolation behavior may deviate from expectations.

Consider the following code snippet:

<code class="php">$ha = ['key1' => 'Hello to me'];

print $ha['key1'];   // correct (usual way)
print $ha[key1];     // Warning, works (use of undefined constant)

print "He said {$ha['key1']}"; // correct (usual way)
print "He said {$ha[key1]}";   // Warning, works (use of undefined constant)

print "He said $ha['key1']";   // Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[ key1 ]";   // Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[key1]";     // !! correct (How Comes?)</code>

Note the final line of the code:

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

Surprisingly, this line executes correctly, even though it uses a syntax that is seemingly incorrect. To understand this behavior, we delve into PHP's interpolation mechanism.

PHP allows for variable interpolation using two syntaxes:

1. Double-quoted strings: "$var"
2. Curly braces within double-quoted strings: "${$var}"

In the case of associative arrays, the second syntax is explicitly recommended in the documentation. However, PHP's interpolation mechanism includes a special exception for accessing associative array elements within double-quoted strings.

If a double-quoted variable contains a key not enclosed in curly braces, the interpreter assumes that it refers to an associative array key. This interpretation extends even if the key is separated from the square brackets by spaces or other whitespace characters.

Thus, the following line is recognized as valid interpolation:

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

In conclusion, while it is considered "correct" according to the PHP interpreter, this interpolation behavior may be unexpected. However, the recommended syntax for interpolating associative array elements into strings remains the use of curly braces, as it ensures consistent and predictable behavior across different PHP versions and ensures compatibility with future versions.

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