Using a string as the condition of an if statement in PHP

Question

I am trying to create a function that takes the condition of an if statement as a string and returns it as a boolean to use in an if statement. Once I have the boolean, I want to be able to return $body if is true.

The logic would look something similar to the below:

public function ifStatement($query, $body) {
    if (eval($query)) {
       return $body;
    }
}

So when using the function, the output would look something like this where if the condition is evaluated and if the condition is true, it would return the $body, but if it's false it would return nothing.

ifStatement('true', 'Hello World!');         // Returns 'Hello World!'
ifStatement('false', 'Hello World!');        // Returns nothing
ifStatement('0 == 0', 'Hello World!');       // Returns 'Hello World!'
ifStatement('0 == 1', 'Hello World!');       // Returns nothing

The primary reason I am looking for something like this is so because I am creating a template engine where I want to be able to pass if statements and other logic statements into php as a string and have it return the appropriate values.

Answer 1

Using eval to evaluate the string like that will not work because it is not valid php syntax. Instead of using eval($query), try using eval("return $query;") that way you are actually returning the value as a boolean instead of using the string itself in the condition of the if statement.

So the revised code would look something like this:

function ifStatement($query, $body) {

    $condition = eval("return $query;");
    
    if ($condition) return $body;
    return;
}

In that new function you are able to take in whatever condition you want and it will be evaluated as a boolean. So then the resulting outputs would like like this:

print ifStatement("true", "This is true");
    # Returns "This is true"

print ifStatement("false", "This is false");
    # Returns nothing

print ifStatement("1 == 1", "1 is 1");
    # Returns "1 is 1"

print ifStatement("1 == 2", "1 is not 1");
    # Returns nothing

---

Adding a false condition

If you wanted to go a step further, you could modify the function a little bit more to include what is returned in the event that the condition is false. All you would need to do is add a new optional param which would be what is returned if the condition is false. It would look like this:

function ifStatement($query, $trueBody, $falseBody = "") {

    $condition = eval("return $query;");
    
    if ($condition) return $trueBody;
    else return $falseBody;
}

With the outputs looking like this:

print ifStatement("true", "This is true", "This is false");
    # Returns "This is true"

print ifStatement("false", "This is true", "This is false");
    # Returns "This is false"

print ifStatement("1 == 1", "1 is 1", "1 is not 1");
    # Returns "1 is 1"

print ifStatement("1 == 2", "1 is 1", "1 is not 1");
    # Returns "1 is not 1"