<?php
    // Here's a string that isn't "empty", it's zero
    $string = '0';
    if (empty($string)) {
        echo 'Oops!';
    }
?>
 
Oops!

<?php
function test_strlen_vs_empty($string) {
    /* We occasionally have oddities so I wanted to make them observable and count them properly so our avg is accurate */
 
    $oddities = 0;
 
    /* We want to be able to test an unset variable */
    if ($string == 'unset') {
        unset($string);
    }   
 
    /* We'll do 100 tests */
    for ($i = 0; $i < 100; $i++) {
        /* Start our first timer */
        $start_time = microtime(true);
 
        /* We want to check each function 100 times */
        for ($ii = 0; $ii < 100; $ii++) {
            /* Check if the string is empty() */
            if (empty($string)) {
            }   
        }
        /* End our first timer */
        $end_time = microtime(true);
 
        /* Calculate how long it took */
        $time_for_empty = $end_time - $start_time;
 
        /* Start our second timer */
        $start_time = microtime(true);
        for ($ii = 0; $ii < 100; $ii++) {
            /* Check if strlen() returns something equal to false */
            if (!strlen($string)) {
            }   
        }
 
        /* End our second timer */
        $end_time = microtime(true);
 
        /* Calculate how long it took */
        $time_for_strlen = $end_time - $start_time;
 
        /* If strlen() was faster (which doesn't happen often) we want to record it */
        if ($time_for_strlen < $time_for_empty) {
            $oddities++;
        } else {
            /* Otherwise, it's as normal. We want to grab a nice percentage of how much faster empty was than strlen */
            $slowers_total += round(100 - (($time_for_empty / $time_for_strlen) * 100), 2); 
        }   
    }   
    /* Output the results with an average taking into account any oddities that may have occurred */
    echo "\n" . 'Average speed gain using empty() instead of strlen(): ' . round($slowers_total / (100 - $oddities), 2) . '%';
    if ($oddities) {
        echo "\n" . 'Number of times where strlen() was actually faster: ' . $oddities;
    }   
}
 
echo "\n\n" . 'Checking "if (!strlen($string))" VS "if (empty($string))"' . "\n\n";
echo 'unset($string);';
test_strlen_vs_empty('unset');
echo "\n\n" . '$string === false;';
test_strlen_vs_empty(false);
echo "\n\n" . '$string = \'\';';
test_strlen_vs_empty('');
?>

function clean_string_for_valid_xml($string) {
    $entities_array = array();
    foreach (get_html_translation_table(HTML_ENTITIES, ENT_QUOTES) as $character => $entity) {
        $entities_array[$entity] = '&#' . ord($character) . ';';
    }
    return str_replace(array_keys($entities_array), $entities_array, $string);
}

clean_string_for_xhtml($string) {
    $entities_array = array();
    foreach (get_html_translation_table(HTML_ENTITIES, ENT_QUOTES) as $character => $entity) {
        $entities_array[$entity] = '&#' . ord($character) . ';';
    }
    $entities_array += array ('''=>''', '−'=>'-', 'ˆ'=>'^', '˜'=>'~', 'Š'=>'Š', '‹'=>'‹', 'Œ'=>'Œ', '‘'=>'‘' , '’'=>'’', '“'=>'“', '”'=>'”', '•'=>'•', '–'=>'–', '—'=>'—', '˜'=>'˜', '™'=>'™', 'š'=>'š', '›'=>'›', 'œ'=>'œ', 'Ÿ'=>'Ÿ', 'ÿ'=>'ÿ', 'Œ'=>'Œ', 'œ'=>'œ', 'Š'=>'Š', 'š'=>'š', 'Ÿ'=>'Ÿ', 'ƒ'=>'ƒ', 'ˆ'=>'ˆ', '˜'=>'˜', 'Α'=>'Α', 'Β'=>'Β', 'Γ'=>'Γ', 'Δ'=>'Δ', 'Ε'=>'Ε', 'Ζ'=>'Ζ', 'Η'=>'Η', 'Θ'=>'Θ', 'Ι'=>'Ι', 'Κ'=>'Κ', 'Λ'=>'Λ', 'Μ'=>'Μ', 'Ν'=>'Ν', 'Ξ'=>'Ξ', 'Ο'=>'Ο', 'Π'=>'Π', 'Ρ'=>'Ρ', 'Σ'=>'Σ', 'Τ'=>'Τ', 'Υ'=>'Υ', 'Φ'=>'Φ', 'Χ'=>'Χ', 'Ψ'=>'Ψ', 'Ω'=>'Ω', 'α'=>'α', 'β'=>'β', 'γ'=>'γ', 'δ'=>'δ', 'ε'=>'ε', 'ζ'=>'ζ', 'η'=>'η', 'θ'=>'θ', 'ι'=>'ι', 'κ'=>'κ', 'λ'=>'λ', 'μ'=>'μ', 'ν'=>'ν', 'ξ'=>'ξ', 'ο'=>'ο', 'π'=>'π', 'ρ'=>'ρ', 'ς'=>'ς', 'σ'=>'σ', 'τ'=>'τ', 'υ'=>'υ', 'φ'=>'φ', 'χ' =>'χ', 'ψ'=>'ψ', 'ω'=>'ω', 'ϑ'=>'ϑ', 'ϒ'=>'ϒ', 'ϖ'=>'ϖ', ' '=>' ', ' '=>' ', ' '=>' ', '‌'=>'‌', '‍'=>'‍', '‎'=>'‎', '‏'=>'‏', '–'=>'–', '—'=>'—', '‘'=>'‘', '’'=>'’', '‚'=>'‚', '“'=>'“',     '”'=>'”', '„'=>'„', '†'=>'†', '‡'=>'‡', '•'=>'•', '…'=>'…', '‰'=>'‰', '′'=>'′', '″'=>'″', '‹'=>'‹', '›'=>'›', '‾'=>'‾', '⁄'=>'⁄', '€'=>'€', 'ℑ'=>'ℑ', '℘'=>'℘', 'ℜ'=>'ℜ', '™'=>'™', 'ℵ'=>'ℵ', '←'=>'←', '↑'=>'↑', '→'=>'→', '↓'=>'↓', '↔'=>'↔', '↵'=>'↵', '⇐'=>'⇐', '⇑'=>'⇑', '⇒'=>'⇒', '⇓'=>'⇓', '⇔'=>'⇔', '∀'=>'∀', '∂'=>'∂', '∃'=>'∃', '∅'=>'∅', '∇'=>'∇', '∈'=>'∈', '   ∉'=>'∉', '∋'=>'∋', '∏'=>'∏', '∑'=>'∑', '−'=>'−', '∗'=>'∗', '√'=>'√', '∝'=>'∝', '∞'=>'∞', '∠'=>'∠', '∧'=>'∧', '∨'=>'∨', '∩'=>'∩', '∪'=>'∪', '∫'=>'∫', '∴'=>'∴', '∼'=>'∼', '≅'=>'≅', '≈'=>'≈', '≠'=>'≠', '≡'=>'≡', '≤'=>'≤', '≥'=>'≥', '⊂'=>'⊂', '⊃'=>'⊃', '⊄'=>'⊄', '⊆'=>'⊆', '⊇'=>'⊇', '⊕'=>'&    #8853;', '⊗'=>'⊗', '⊥'=>'⊥', '⋅'=>'⋅', '⌈'=>'⌈', '⌉'=>'⌉', '⌊'=>'⌊', '⌋'=>'⌋', '⟨'=>'〈', '⟩'=>'〉', '◊'=>'◊', '♠'=>'♠', '♣'=>'♣', '♥'=>'♥', '♦'=>'♦');
 
    return str_replace(array_keys($entities_array), $entities_array, $string);
}