Unable to get var value inside function

Question

I am trying to get field_1 value into the input text and I tried this code but it doesn't work.

function bbg_add_reg_field() {
  $field_1= $_POST["signup_username"];  
  echo '<input name="field_1" id="field_1" value="$field_1" type="text">';  
}
add_action( 'bp_before_registration_submit_buttons', 'bbg_add_reg_field' );

The echo value shown inside the input text is $field_1

The value should be a name, say for example peter.

Answer 1

In PHP you can interpolate a string (add variables to a string with $var) only when it is a double quoted string.

From the PHP Documentation

// Outputs: Variables do not $expand $either
echo 'Variables do not $expand $either';

Instead you could do this

echo '<input name="field_1" id="field_1" value="' . $field_1 . '" type="text">';

oh and don't forget htmlspecialchars

echo '<input name="field_1" id="field_1" value="' . htmlspecialchars($field_1) . '" type="text">';

You could also achieve the same using PHP Heredoc. Which goes like this

echo <<<EOF
<input name="field_1" id="field_1" value="$field_1" type="text">
EOF;