Function naming (underscores)
Even between similar/related functions, some use underscores between words, while others do not. Here are some examples:
get: php gettype php get_class
str: php str_ireplace php str_pad php str_repeat php str_replace php str_shuffle php str_split php str_word_count php strcasecmp php strchr php strcmp php strcoll php strcspn
encode: php base64_encode php quoted_printable_encode php session_encode php rawurlencode php urlencode php gzencode
php: php php_uname php php_sapi_name php php_logo_guid php phpinfo php phpcredits php phpversion
htmlentites: php htmlentities php html_entity_decode
Significance: Consistency
Language consistency is very important for developer efficiency. Every inconsistent language feature means that developers have one more thing to remember, one more reason to rely on the documentation, or one more situation that breaks their focus. A consistent language lets developers create habits and expectations that work throughout the language, learn the language much more quickly, more easily locate errors, and have fewer things to keep track of at once.