How to Determine If a Username Contains Only Lowercase Letters Using the ctype_lower Function?

<?php  
// Beginning of article (not part of the main content)  
echo "This is an article about the application of the ctype_lower function in PHP.";  
?>  
<p><hr></p>
<p><?php<br>
// Main content of the article</p>
<p>/*</p>
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<p>How to determine if a username contains only lowercase letters using the ctype_lower function?</p>
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<p>In PHP, validating whether a string contains only lowercase letters is a common requirement,</p>
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<p>especially in scenarios like username validation.</p>
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<p>PHP provides a very useful built-in function, ctype_lower,</p>
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<p>which can quickly determine whether a string consists solely of lowercase letters.</p>
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<p>Below we’ll explain in detail how to use the ctype_lower function</p>
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<p>and how to apply it to check if a username contains only lowercase letters.<br>
*/</p>
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<p>/**</p>
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<p>About the ctype_lower function:</p>
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<p>ctype_lower(string $text): bool</p>
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<p>Returns true only if all characters in $text are lowercase letters (a-z).</p>
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<p>Returns false if the string is empty.</p>
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<p>Any non-letter characters (like numbers or symbols) will cause the function to return false.</p>
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<p>Examples:</p>
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<p>ctype_lower("hello")   returns true</p>
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<p>ctype_lower("Hello")   returns false (contains uppercase letters)</p>
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<p>ctype_lower("hello123") returns false (contains numbers)<br>
*/</p>
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<p>function isUsernameValid(string $username): bool {<br>
// Check if the string contains only lowercase letters<br>
// Using ctype_lower function and ensuring the username is not empty<br>
if (empty($username)) {<br>
return false;<br>
}<br>
return ctype_lower($username);<br>
}</p>
<p>// Test examples<br>
$usernames = [<br>
"alice",      // Valid: all lowercase letters<br>
"Alice",      // Invalid: contains uppercase letters<br>
"bob123",     // Invalid: contains numbers<br>
"charlie_",   // Invalid: contains underscore<br>
"",           // Invalid: empty string<br>
];</p>
<p>foreach ($usernames as $name) {<br>
if (isUsernameValid($name)) {<br>
echo "Username '{$name}' is valid and contains only lowercase letters.<br>";<br>
} else {<br>
echo "Username '{$name}' is invalid and contains non-lowercase characters.<br>";<br>
}<br>
}</p>
<p>/*</p>
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<p>Summary:</p>
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<p>ctype_lower is a simple and efficient way to check whether a string contains only lowercase letters.</p>
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<p>When validating usernames, it can be combined with other rules (like length and character set restrictions)</p>
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<p>to ensure proper formatting and security.<br>
*/<br>
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