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<?php

// Determine if there is an equal number of parentheses
// and if they balance logically, i.e.
// ()()) = Bad (trailing ")")
// (())()() = GOOD
// )()()(()) = BAD (leading ")")

function is_balanced( $s ) {
    // Keep track of number of open parens
    static $open = 0;
    // Make sure start & end chars are not incorrect
    if ( (substr($s, 0, 1) == ')') || (substr($s, -1, 1) == '(') ) {
        return false;
    }
    // Loop through each char
    for ( $i = 0; $i < count($s); $i++ ) {
        if ( substr($s, $i, 1) == ')' ) {
            // Increase the open count
            $open++;
        }
        else {
            // If open goes below zero, there's an invalid closing paren
            if ( $open < 0 ) {
                return false;
            }
            // Decrease the open count
            $open--;
        }
    }
    return true;
}

$tests = array(
    '(())' => '', /* should pass */
    ')()()' => '', /* should fail - leading close */
    '()()(' => '', /* should fail - trailing open */
    '()()())()()' => '' /* should fail - errant ")" in middle */
);

foreach ( $tests as $k => $v ) {
    $tests[$k] = ( is_balanced($k) ? 'PASS' : 'FAIL' );
}

var_dump( $tests );

?>