How to check if letter is upper or lower in PHP?

function starts_with_upper($str) {
    $chr = mb_substr ($str, 0, 1, "UTF-8");
    return mb_strtolower($chr, "UTF-8") != $chr;
}

Note that mb_substr is necessary to correctly isolate the first character.

Use ctype_upper for check upper case:

$a = array("Word", "word", "wOrd");

foreach($a as $w)
{
    if(ctype_upper($w{0}))
    {
        print $w;
    }
}

It is my opinion that making a preg_ call is the most direct, concise, and reliable call versus the other posted solutions here.

echo preg_match('~^\p{Lu}~u', $string) ? 'upper' : 'lower';

My pattern breakdown:

~      # starting pattern delimiter 
^      #match from the start of the input string
\p{Lu} #match exactly one uppercase letter (unicode safe)
~      #ending pattern delimiter 
u      #enable unicode matching

Please take notice when ctype_ and < 'a' fail with this battery of tests.

Code: (Demo)

$tests = ['âa', 'Bbbbb', 'Éé', 'iou', 'Δδ'];

foreach ($tests as $test) {
    echo "\n{$test}:";
    echo "\n\tPREG:  " , preg_match('~^\p{Lu}~u', $test)      ? 'upper' : 'lower';
    echo "\n\tCTYPE: " , ctype_upper(mb_substr($test, 0, 1))  ? 'upper' : 'lower';
    echo "\n\t< a:   " , mb_substr($test, 0, 1) < 'a'         ? 'upper' : 'lower';

    $chr = mb_substr ($test, 0, 1, "UTF-8");
    echo "\n\tMB:    " , mb_strtoupper($chr, "UTF-8") == $chr ? 'upper' : 'lower';
}

Output:

âa:
    PREG:  lower
    CTYPE: lower
    < a:   lower
    MB:    lower
Bbbbb:
    PREG:  upper
    CTYPE: upper
    < a:   upper
    MB:    upper
Éé:               <-- trouble
    PREG:  upper
    CTYPE: lower  <-- uh oh
    < a:   lower  <-- uh oh
    MB:    upper
iou:
    PREG:  lower
    CTYPE: lower
    < a:   lower
    MB:    lower
Δδ:               <-- extended beyond question scope
    PREG:  upper  <-- still holding up
    CTYPE: lower
    < a:   lower
    MB:    upper  <-- still holding up

If anyone needs to differentiate between uppercase letters, lowercase letters, and non-letters see this post.

---

It may be extending the scope of this question too far, but if your input characters are especially squirrelly (they might not exist in a category that Lu can handle), you may want to check if the first character has case variants:

\p{L&} or \p{Cased_Letter}: a letter that exists in lowercase and uppercase variants (combination of Ll, Lu and Lt).

• Source: https://www.regular-expressions.info/unicode.html

To include Roman Numerals ("Number Letters") with SMALL variants, you can add that extra range to the pattern if necessary.

https://www.fileformat.info/info/unicode/category/Nl/list.htm

Code: (Demo)

echo preg_match('~^[\p{Lu}\x{2160}-\x{216F}]~u', $test) ? 'upper' : 'not upper';