Generate a random math question that always has a positive answer

random_math_question_positive_only.php

<?php

/**
 * Starting with an answer, generate a math question to get the result
 * @param int $answer Your desired answer
 * @return string The equation to arrive at that answer
 */
function generate_simple_math($answer){
    $operations = 2;
    $min_number = 1;
    $temp_answer = $answer;
    $op_convert = array('+'=>'-', '-'=>'+');
    $ops = array('-', '+');
    $operators = array();
    for($i = 0; $i < $operations; $i++){
        $operators[] = $ops[rand(0,1)];
    }
    $question = array();
    $numbers = array();
    for($i = 0; $i < $operations; $i++){
        $max_number = $temp_answer - 1;
        $num = rand($min_number,$max_number);
        $numbers[] = $num;
        $temp_answer = eval('return ' . $temp_answer . ' ' .$operators[$i] . ' ' . $num. ';');
        //push the values onto the stack in the correct order
        $question[] = $op_convert[$operators[$i]];
        $question[] = $num;
    }

    //add 1 to one of the other operands, set temp_answer to be -1 because 0s are hard apparently
    if($temp_answer == 0){
        $question[1] = $question[1] + 1;
        $temp_answer = -1;

    }

    //push the remainder on to the front
    if($temp_answer > 0){
        array_unshift($question, $temp_answer);
    //put the remainder on the end if it is less than 0 because starting with a negative is hard
    } else {
        //first operator is the one that is accociated with the
        $op = array_shift($question);
        $question[] = $op;
        $question[] = $temp_answer;
    }
    $question = implode($question, '');
    //might have double operators because of "no zeros" requirement
    $return_question = str_replace(array('--', '-+', '+-', '++'), array('+', '-', '-', '+'), $question);
    return $return_question;
}

//no problems in 100000 iterations... probably safe... but not for sure
for($i = 0; $i < 100000; $i++){
    $min_answer = 1;
    $max_answer = 15;

    $answer = rand($min_answer,$max_answer);
    $question = generate_simple_math($answer);

    $result = eval('return '.$question.';');
    //major error here... this should be investigated
    if($result !== $answer){
        $var_dump = array(
            'question'=>$question,
            'answer'=>$answer,
            'eval_result'=>$result,
        );
        print_r($var_dump);
        die();
    }
    if(strpos($question , '0') !== false){
        $var_dump = array(
            'question'=>$question,
            'answer'=>$answer,
            'eval_result'=>$result,
        );
    }
}

this is a bad idea... don't use this for anything... it just barely works