eval - process embedded variable within a string using PHP -

how process variable embedded within string? string came database, here example:

1: $b='world!'; 2: $a='hello $b';   #note, used single quote purposely emulate string database (i know different of using ' or "). 3: eval($c=$a.";"); #i know not work, trying $c="hello $b"; #with line 3 php code, trying outcome of $c='hello world!'; 

if want eval line of code $c='hello world'; should have string when echoed that: $c="hello $b";.

so - start, $c variable should inside string (and not variable);

'$c' 

next - = sign should inside string (and not part of php code, otherwise preprocessor try assign value on right variable on left.

how one:

$new_str = '$c=' . $a . ';'; echo $new_str; 

now can see value inside $new_str actually:

$c=hello $b; 

which not valid php code (because don't have hello in php. want have hello $b part inside double-quote:

$new_str = '$c="' . $a . '";'; 

and can eval this.

so final code should like:

$b='world!'; $a='hello $b';  eval('$c="' . $a . '";'); echo $c; // hello world!