Replace substr($a, 0, $i) with str_starts_with()

Problem/Motivation

Part of #3324560: Replace strpos/substr with PHP 8's str_starts_with() / str_contains() / str_ends_with().

Proposed resolution

1. Replace most substr($a, 0, $i) == $b with str_starts_with($b).
2. A few are deliberately omitted where they would cause merge conflicts with other child issues.

Remaining tasks

• When reviewing, consider using git diff --color-words or git diff --porecelain.
• Pay attention to the fact that $i should be the length of $b.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A