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PHP variable variables have unexpected behavior when used with arrays because they are not intended to be used with arrays. Variable variables provide a way to dynamically create variable names by using the value of another variable as the name. However, when used with arrays, the variable name is interpreted as a string, which is not the intended behavior.

For example, consider the following code:

$my_array = [
    'foo' => 'hello',
    'bar' => 'world'
];

$var_name = 'my_array';

echo $$var_name['foo'];

The expected output would be "hello", since we are trying to access the "foo" element of the "myarray" array using a variable variable. However, the actual output is "m", because the expression $$varname['foo'] is interpreted as ${$varname}['foo']. Here, $varname is the string "my_array", and when we try to access the 'foo' element of this string, we get the first character ('m') instead.

To avoid this unexpected behavior, it is recommended to use standard array syntax instead of variable variables when working with arrays in PHP.